The Evolution of Man, V.2 by Ernst Haeckel (leveled readers .txt) 📖

head-coelom, which only combine at a later stage. When the lung of the air-breathing Vertebrates issues from the head-cavity and enters the trunk-cavity, it follows the example of the floating bladder of the fishes, which also originates from the pharyngeal wall in the shape of a small pouch-like out-growth, but soon grows so large that, in order to find room, it has to pass far behind into the trunk-cavity. To put it more precisely, the lung of the quadrupeds retains this hereditary growth-process of the fishes; for the hydrostatic floating bladder of the latter is the air-filled organ from which the air-breathing organ of the former has been evolved.

There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice. In its earliest form the heart is DOUBLE, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we took as our starting-point, is only formed at a later stage, when the two lateral tubes move backwards, touch each other, and at last combine in the middle line. In man, as in the rabbit, the two embryonic hearts are still far apart at the stage when there are already eight primitive segments (Figure 1.134 h). So also the two coelom-pouches of the head in which they lie are still separated by a broad space. It is not until the permanent body of the embryo develops and detaches from the embryonic vesicle that the separate lateral structures join together, and finally combine in the middle line. As the median partition between the right and left cardiocoel disappears, the two cervical cavities freely communicate (Figure 2.381), and form, on the ventral side of the amniote head, a horseshoe-shaped arch, the points of which advance backwards into the pleuro-ducts or pleural cavities, and from there into the two peritoneal sacs of the trunk. But even after the conjunction of the cervical cavities (Figure 2.381) the two cardiac tubes remain separate at first; and even after they have united a delicate partition in the middle of the simple endothelial tube (Figures 2.379 s and 2.382 h) indicates the original separation. This CENOGENETIC "primary cardiac septum" presently disappears, and has no relation to the subsequent permanent partition between the halves of the heart, which, as a heritage from the reptiles, has a great PALINGENETIC importance.

Thorough opponents of the biogenetic law have laid great stress on these and similar cenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law. As in every other instance, careful, discriminating, comparative-morphological examination converts these supposed disproofs of evolution into strong arguments in its favour. In his excellent work, On the structure of the Heart in the Amphibia (1886), Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk.

The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny. But as one needs a thorough knowledge of the intricate structure of the whole vascular system in man and the other Vertebrates in order to follow this with profit, we cannot go into it further here. Moreover, many important features in the ontogeny of the vascular system are still very obscure and controverted. The characters of the embryonic circulation of the Amniotes, which we have previously considered (

Chapter 1.

15), are late acquisitions and entirely cenogenetic. (Cf.

Chapter 1.

15 and Figures 1.198 to 1.202.)

In the Selachii also we find a longitudinal row of segmental canals on each side, which open outwards into the primitive renal ducts (nephrotomes,

Chapter 1.

14). The segmental canals (a pair in each segment of the middle part of the body) open internally by a ciliated funnel into the body-cavity. From the posterior group of these organs a compact primitive kidney is formed, the anterior group taking part in the construction of the sexual organs.

In the same simple form that remains throughout life in the Myxinoides and partly in the Selachii we find the primitive kidney first developing in the embryo of man and the higher Craniotes (Figures 2.386 and 2.387). Of the two parts that compose the comb-shaped primitive kidney the longitudinal channel, or nephroduct, is always the first to appear; afterwards the transverse "canals," the excreting nephridia, are formed in the mesoderm; and after this again the Malpighian capsules with their arterial coils are associated with these as coelous outgrowths. The primitive renal duct, which appears first, is found in all craniote embryos at the early stage in which the differentiation of the medullary tube takes place in the ectoderm, the severance of the chorda from the visceral layer in the entoderm, and the first trace of the coelom-pouches arises between the limiting layers (Figure 2.385). The nephroduct (ung) is seen on each side, directly under the horny plate, in the shape of a long, thin, thread-like string of cells. It presently hollows out and becomes a canal, running straight from front to back, and clearly showing in the transverse section of the embryo its original position in the space between horny plate (h), primitive segments (uw), and lateral plates (hpl). As the originally very short urinary canals lengthen and multiply, each of the two primitive kidneys assumes the form of a half-feathered leaf (Figure 2.387). The lines of the leaf are represented by the urinary canals (u), and the rib by the outlying nephroduct (w). At the inner edge of the primitive kidneys the rudiment of the ventral sexual gland (g) can now be seen as a body of some size. The hindermost end of the nephroduct opens right behind into the last section of the rectum, thus making a cloaca of it. However, this opening of the nephroducts into the intestine must be regarded as a secondary formation. Originally they open, as the Cyclostomes clearly show, quite independently of the gut, in the external skin of the abdomen.

(FIGURE 2.395. Primitive kidneys and germinal glands of a human embryo, three inches in length (beginning of the sixth week), magnified fifteen times. k germinal gland, u primitive kidney, z diaphragmatic ligament of same, w Wolffian duct (opened on the right), g directing ligament (gubernaculum), a allantoic duct. (From Kollmann.))

In the Myxinoides the primitive kidneys retain this simple comb-shaped structure, and a part of it is preserved in the Selachii; but in all the other Craniotes it is only found for a short time in the embryo, as an ontogenetic reproduction of the earlier phylogenetic structure. In these the primitive kidney soon assumes the form (by the rapid growth, lengthening, increase, and serpentining of the urinary canals) of a large compact gland, of a long, oval or spindle-shaped character, which passes through the greater part of the embryonic body-cavity (Figures 1.183 m, 1.184 m, 2.388 n). It lies near the middle line, directly under the primitive vertebral column, and reaches from the cardiac region to the cloaca. The right and left kidneys are parallel to each other, quite close together, and only separated by the mesentery--the thin narrow layer that attaches the middle gut to the under surface of the vertebral column. The passage of each primitive kidney, the nephroduct, runs towards the back on the lower and outer side of the gland, and opens in the cloaca, close to the starting-point of the allantois; it afterwards opens into the allantois itself.

(FIGURES 2.396 TO 2.398. Urinary and sexual organs of ox-embryos. Figure 2.396, female embryo one and a half inches long; Figure 2.397, male embryo, one and a half inches long. Figure 2.398 female embryo two and a half inches long. w primitive kidney, wg Wolffian duct, m Mullerian duct, m apostrophe upper end of same (opened at t), i lower and thicker part of same (rudiment of uterus), g genital cord, h testicle, (h apostrophe, lower and h double apostrophe, upper testicular ligament), o ovary, o apostrophe lower ovarian ligament, i inguinal ligament of primitive kidney, d diaphragmatic ligament of primitive kidney, nn accessory kidneys, n permanent kidneys, under them the S-shaped ureters, between these the rectum, v bladder, a umbilical artery. (From Kolliker.))

The primitive or primordial kidneys of the amniote embryo were formerly called the "Wolffian bodies," and sometimes "Oken's bodies." They act for a time as kidneys, absorbing unusable juices from the embryonic body and conducting them to the cloaca--afterwards to the allantois. There the primitive urine accumulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and the other Amniotes. It has, however, no genetic connection with the primitive kidneys, but is a pouch-like growth from the anterior wall of the rectum (Figure 1.147 u). Thus it is a product of the visceral layer, whereas the primitive kidneys are a product of the middle layer. Phylogenetically we must suppose that the allantois originated as a pouch-like growth from the cloaca-wall in consequence of the expansion caused by the urine accumulated in it and excreted by the kidneys. It is originally a blind sac of the rectum. The real bladder of the vertebrate certainly made its first appearance among the Dipneusts (in Lepidosiren), and has been transmitted from them to the Amphibia, and from these to the Amniotes. In the embryo of the latter it protrudes far out of the not yet closed ventral wall. It is true that many of the fishes also have a "bladder." But this is merely a local enlargement of the lower section of the nephroducts, and so totally different in origin and composition from the real bladder. The two structures can be compared from the physiological point of view, and so are ANALOGOUS, as they have the same function; but not from the morphological point of view, and are therefore not HOMOLOGOUS. The false bladder of the fishes is a mesodermic product of the nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of the rectum.

In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower stage of development to this extent, that the primitive kidneys (protonephri) act permanently as urinary glands. This is only so as a passing phase of the early embryonic life in the three higher classes of Vertebrates, the Amniotes. In these the permanent or after or secondary (really tertiary) kidneys (renes or metanephri) that are distinctive of these three classes soon make their appearance. They represent the third and last generation of the vertebrate kidneys. The permanent kidneys do not arise (as was long supposed) as independent glands from the alimentary tube, but from the last section of the primitive kidneys and the nephroduct. Here a simple tube, the secondary renal duct, develops, near the point of its entry into the cloaca; and this tube grows considerably forward. With its blind upper or anterior end is connected a glandular renal growth, that owes its origin to a differentiation of the last part of the primitive kidneys. This rudiment of the permanent kidneys consists of coiled urinary canals with Malpighian capsules and vascular coils (without ciliated funnels), of the same structure as the segmental mesonephridia of the primitive kidneys. The further growth of these metanephridia gives rise to the compact permanent kidneys, which have the familiar bean-shape in man and most of the higher mammals, but consist of a number of separate folds in the lower mammals, birds, and reptiles. As the permanent kidneys grow rapidly and advance forward, their passage, the ureter, detaches altogether from its birth-place, the posterior end of the nephroduct; it passes to the posterior surface of the allantois. At first in the oldest Amniotes this ureter opens into the cloaca together with the last section of the nephroduct, but afterwards separately from this, and finally into the permanent bladder apart from the rectum altogether. The bladder originates from the hindmost and lowest part of the allantoic pedicle (urachus), which enlarges in spindle shape before the