Book - Vertebrate Zoology (1928) 27

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXVII The Excretory and Reproductive Systems

Amphioxus is unique among chordate animals in possessing true nephridia. These organs are situated above the gill-slits, their solenocytes project into the lateral dorsal coelomic cavities, and their external openings lead into the atrial cavity. The gonads of Amphioxus are segmental, and situated at the ventral ends of the original myoccelic cavities. The germ-cells of each segment make their way independently to the exterior (actually into the atrial cavity) by pores in the body- wall.

It is possible that the region of the ccelom, which in all higher Chordates is concerned with the formation of the ex- cretory organs, corresponds to that region which in Amphioxus forms the gonads.

Originally there must have been a continuous row of little tubes on each side of the body, leading out of the splanchnocoel into a duct which collected from them all, and opened to the outside at or near the anus. These little tubes represent the original connexion between the myoccel and the splanchnocoel (the nephrocoel, in the intermediate cell-mass), and consequently they are segmental in arrangement : one tubule on each side to each segment. Such an arrangement has been called an archinephros, and the duct the archinephric duct, and this condition is almost fulfilled in the Cyclostome Bdellostoma. Here a continuous row of tubules is formed, but an inter- mediate section of them disappears, thus separating an anterior batch — the pronephros — from a more posterior set — the mesonephros. In other forms the pronephros appears first, and the duct which is formed by the backward growth of the ends of the tubules is the pronephric duct. The pronephros is the functional larval kidney in the lower vertebrates, and the pronephric duct grows back to the cloaca without waiting for the mesonephric tubules to develop. When these form, they find the pronephric duct ready-made to receive them. After receiving the mesonephric tubules the pronephric duct becomes known as the mesonephric duct, and the pronephros degenerates (except in the bony fish Fierasfer and Gobiesox). The functional kidney in the adult fish or amphibian is the mesonephros.

The cavity of each tubule (pronephric or mesonephric) becomes shut off from the splanchnocoel, although the opening of the tubule into the splanchnocoel (the ciliated funnel or ccelomostome) may persist (as in Selachians and amphibia) on the median side of the occlusion. The cavity of the tubule now becomes known as a Bowman's capsule, and its wall is indented by capillaries from the dorsal aorta and leading to the posterior cardinal vein, forming the glomerulus. Bowman's capsule and the glomerulus together form a Malpighian corpuscle. Primitively, these corpuscles are segmen tally arranged, and this condition is retained in Myxine. In other forms the number of Malpighian corpuscles is greatly increased by the formation of others by budding.

In the Cyclostomes, the germ-cells in the two sexes are shed into the coelomic cavity, and make their way to the exterior by a pair of pores at the base of the mesonephric ducts. In all higher forms the sperms are never shed into the ccelom, but led by vasa efferentia to the vas deferens, primitively passing through the tubules of the mesonephros. The vasa efferentia are the ccelomic funnels leading into the tubules, and the mesonephric duct forms the vas deferens or Wolffian duct. In addition, on each side there is another duct, in the embryo. This is the Miillerian duct or oviduct, which develops in the females but becomes reduced in the males. The Miillerian duct in the Selachians arises by splitting off from the Wolffian duct, but in other forms it grows back independently from its opening into the ccelom (the oviducal funnel or Fallopian tube) in front, to the cloaca behind. The eggs then are shed into the ccelom whence they enter the oviducts, whereas the sperms pass down a duct which serves for them as well as for the evacuation of urine from the kidney. Thus, while the Cyclostome has a single kidney-duct on each side in both sexes, and the germ-cells do not pass through it, in the fish and amphibia typically the females have two ducts on each side. One of these is the Wolffian duct evacuating the urine, the other is the Mullerian duct leading out the eggs. In the males of fish and amphibia the Wolffian duct evacuates both urine and sperms ; the Mullerian duct is reduced, and in the Selachian is represented only by the funnel and the sperm- sacs. This condition is typically represented in the frog and newt.

In several different groups of fish and amphibia, this arrangement is slightly altered by the separation of a part of the Wolffian duct conveying the sperms (vas deferens) from another part which drains the kidney (mesonephric ureter, not a true ureter). By this means, the sperms avoid going through the excretory part of the kidney, and this condition is found in the Dipnoan Protopterus, Polypterus, the Teleosts, and in such toads as Alytes, in all of which it has been inde- pendently developed. In Scyllium, it will be remembered that only the hinder part of the mesonephros is excretory in function, and the sperms pass through the anterior part.

In Lepidosteus and many Teleosts, the ccelomic wall surrounds the ovary forming a sac which joins on to the oviduct. In this manner the ovary is completely shut off from the ccelomic cavity, and consequently the eggs are not shed into it, but led directly to the exterior.

In the amniotes, the functional kidney in the adult is the metanephros, and the metanephric duct or ureter is an out- growth from the Wolffian duct. The Wolffian duct is therefore spared the function of evacuting urine, and it persists only in the male, where it functions solely as a vas deferens for the sperm. The mesonephric tubules form the epididymis. The Mullerian duct disappears in the male, and the Wolffian duct disappears in the female. The Mullerian duct persists in the female as the oviduct. In the adult bird, only the left ovary and oviduct persist.

Except in the Monotremes, the base of the oviduct in the mammals becomes specialised to form the uterus, in which the embryos undergo development. According as to whether the bases of the two oviducts remain separate or become fused together the uterus may be double or single.

Another peculiarity of the mammalian reproductive system, is the fact that in the male, the testes usually leave their position in the roof of the abdominal cavity, and descend into scrotal sacs (see p. 150).

While claspers or copulatory organs are present in the males of several fish, the amphibia lack them (except the Gymnophiona), and fertilisation has to take place in water since the sperm require a fluid medium. In Anura the eggs and sperm are shed together into the water. In the newts, as a rule, the male lays a packet of sperm, and then gives a display of " courtship " in front of the female to stimulate her to pick up the packet with her pelvic limbs and place it in her cloaca. During the breeding season the male has specially developed secondary sexual (epigamic) characters, such as the crest and the colour of the belly, which assist in the courtship activities.

In the amniotes, fertilisation is internal, and the sperms are introduced into the cloaca of the female by the copulatory organ or penis of the male. In this way, the amniotes are independent of water for fertilisation.

A feature of considerable interest is the increase in care of the young after they are hatched, by the parents. This increases in the higher groups of vertebrates, and all stages can be found in the evolution of the family, from the condition of Amphioxus where fertilisation takes place in the sea water outside the parents which are in no way concerned with the development of the young, to that of man. This evolution has involved the development and perfection of characters of behaviour as well as those of structure. The first step in this direction is usually the habit of protecting the eggs until the young hatch. In several species of fish, the eggs are laid in holes or in nests specially prepared by the parents, and the male remains on guard. This habit is resorted to by Proto-pterus and Lepidosiren among the Dipnoi, by Amia, several catfish, and the stickleback, only to mention a few. In some of these cases there are interesting adaptations for ensuring a sufficient supply of oxygen to the eggs. So in Lepidosiren, the pelvic fin of the male becomes modified into a tuft-like organ well supplied with blood, from which oxygen diffuses out into the water. In some catfish, the eggs are carried about by the parent (usually the male), and so are continually exposed to fresh sea water. Ichthyophys (Gymnophiona) coils itself round its eggs in a burrow, as do some snakes such as the python. Several Anura lay their eggs in nests specially prepared ; others make living nests of themselves. In Pipa the eggs are placed on the female's back, where they sink into pits and undergo development ; the male Rhinoderma carries the eggs in large vocal sacs ; Hylambates carries the eggs in its mouth. Alytes is peculiar in that pairing takes place on land, and the eggs, which are tied together by strings of slime, are carried about by the male, wound round his legs. When the young are about to hatch, the male takes them to the water and abandons them. In some viviparous snakes such as the viper, the young remain with the parent for a time. It happens in some forms (e.g. viper) that the egg is hatched while still in the oviduct, without being laid. This condition is called ovo-viviparous.

The eggs of birds require a constant high temperature for their development, and this necessitates the uninterrupted attention of the parents. (In the Megapodes, the eggs are laid in heaps of decaying vegetable matter, the heat of which enables them to incubate.) Nearly all birds lay their eggs in nests, which serve to protect the eggs from enemies, cold and damp. Nests are constructions in which as a rule both sexes take part, and courtship and display play an important part in keeping together the members of a pair to perform the various duties which devolve on them. The male is usually the active partner in courtship, and often possesses brilliant secondary sexual characters used for the purpose. After the nest has been built, these duties include the collecting of food for the sitting partner, and for the young when they hatch in a helpless condition. In some species, this food is freshly- caught, in others it is regurgitated, while in the pigeon the lining of the crop comes away forming a mucous secretion known as " pigeon's milk."

In all mammals, the young are fed on milk produced by the mammary glands of the mother, and they are born alive in all except the Monotr ernes, in which they are hatched from eggs. The mammary glands of the Monotremes are primitive in that they do not have proper teats, but merely exude the milk which is lapped up by the young. In the Monotremes and Marsupials, the ventral surface of the belly of the female is modified to form a pouch, or marsupium. In the Monotreme, the egg is hatched in the pouch. The young Marsupial is born very early after a short period of gestation in the uterus, and in a very undeveloped condition. It becomes attached to a teat of the mammary glands which are situated in the pouch. There it completes its development. In the higher mammals, or Placentals, the allantoic placenta is well developed, and the period of gestation is long. During this time the embryo is able to develop to a high degree of perfection, such as would not be possible without a lasting physiological connexion with the mother in the form of a placenta. After birth the young is supplied with milk by the mother until it is weaned and able to feed for itself. Another feature of the mammals is that they go through a period of " childhood," during which they play, fed by the mother and protected by her and the father. Parental instincts reach their highest form in man, whose superiority in mind and body is conditioned by the relatively very great length of time spent in development, both before and after birth.

There may be one or several Embryos developing in the uterus at the same time. Each embryo develops from a separate egg except in cases of true twinning ; here, as in the armadillo and possibly in man, the twins arise by a process of fission of one blastocyst which has developed from one egg-

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology

Cite this page: Hill, M.A. (2024, June 23) Embryology Book - Vertebrate Zoology (1928) 27. Retrieved from

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