Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/428

Rh 410 REPKODUCTION [ANIMAL. In Myxine and the Lampreys no ducts are present, but the generative organs void their contents into the coelom, whence they pass out by the abdominal pore; some Teleosteans too (Salmonidse) and at least one Elasmobranch (Lxmaryus) exhibit the same primeval simplicity of structure and function. Even here the exit of the sexual products is hardly independent of the aid of the excretory system, since there is reason to regard the abdominal pores as the least modified survivals of segmental organs ; and in all higher forms the definite efferent ducts which are present in more or less close relation to the essential generative organs develop in the closest relation to, and in fact at the expense of, the renal excretory apparatus. For an account of the complex details of this process in the higher Vertebrates, however, the reader must consult the classical monographs of Balfour and Semper, or the larger manuals. (2) Copulation. We have noted above the importance of the copulatory process to secure fertilization of the ovum, and can thus readily understand its occurrence in the higher members of all the more complex animal groups. Though the result is in all cases the same, the process presents curious variations in principle as well as detail. Thus (e.g.} the hermaphrodite Earthworms become firmly attached by their characteristic thickened band of fused rings (ditellum). Among the higher Crustaceans the sper- matozoa are conducted to the ova along the grooves of a modified pair of the appendages of the male, while in In- sects the modifications of the posterior abdominal segments and their limbs for copulatory purposes are often extra- ordinarily complex and varied. In Spiders, again, the spermatic fluid is passed into a receptacle in the chela, and thence pushed into the cloaca. In the higher Mollusca, the complex copulatory apparatus of the Common Snail and the process of hectocotylization among the Cephalo- pods, so curiously analogous to the process in Spiders, are too familiar to need more than mention (see MOLLUSCA, CUTTLE-FISH). In many Fishes no copulatory process exists ; thus in any of our Salmon rivers the male fish can be seen voiding the milt upon the ova after their deposi- tion. In many Elasmobranchs a portion of the posterior pair of limbs, 1 presenting very peculiar cartilaginous and glandular structures, though known as " claspers," seems to be introduced into the cloaca during fertilization. But it is among Amphibians that we find the earliest trace of a true penis ; a portion of the cloaca is distinctly ever- sible in Caecilians ; in Snakes and Lizards paired eversible processes arise from the posterior cloacal wall, while in Chelonians, Crocodiles, and most Birds it is the anterior wall which bears these processes. In Monotremes, too, the organ is distinctly double; in higher Mammals it is single ; but the function is in all cases essentially the same. The nervous, muscular, and circulatory mechanisms of the pro- cess are described in works on human physiology. (3) Gestation and Birth. While in the majority of lower forms the offspring leaves the parent as an unfertilized ovum, we have seen even among Sponges the impregna- tion and development of the embryo in its primitive position, and thus almost from the outset of an ascending zoological survey we can recognize the passage from oviparous to viviparous forms. The Invertebrates, how- ever, are mainly oviparous, despite a few exceptions, of which perhaps the most surprising and aberrant are that of Entoconcha mirabilis, which exhibits an ordinary Molluscan development within the body of its Holothurian host, and that in what resembles a special ovarian tube, but is really the body of its utterly degenerate parasitic parent. Among 1 In the curious Holocephalous Fish Callorhynchus, Jeffery Parker has recently adduced arguments for regarding the claspers as the surviving rudiments of a third pair of limbs. Insects a certain degree of viviparous development may be reached ; and this goes curiously far in the Dipterous Insect Cecidomyi.a, in which larvze develop within the body of their parents (themselves at the larval stage), the cavity of which they destroy and burst in order to become free. Thus within the same species there comes about exactly the state of things in which the ova of a parasite develop at the expense of its host. Among Fishes viviparous birth occurs more commonly ; in some Teleosteans the young develop within the ovaries ; in many Sharks and Dogfish the development takes place within the oviduct, and in one case (Mustdus /am) an actual placenta is formed by the interdigitation of folds of the yolk sac with those of the oviduct. Even the terres- trial Amphibians usually lay their eggs in water, yet in some types, notably the Alpine Salamander (Salamandra atra), development takes place within the oviduct. That this is a clear case of adaptation to the eminently terrestrial environment has indeed been well shown by experiments in which the young larvae taken from the parent and trans- ferred to pond water developed like ordinary Newts. To all such forms, viviparous in the sense of bringing forth their young alive, the somewhat confusing term " ovovivi- parous" is often applied. Birds, and also Reptiles, with few exceptions, of which the Ichthyosaiiria seem to have presented a striking case, are oviparous ; so too, as has been recently established by Caldwell, is the in all respects so curiously Bird-like Mammal Echidna. Its congener Orni- tJwrhynchus probably agrees in this ; but in Marsupials the embryo is not born until it has reached a comparatively advanced state of development, when it is transferred to the brood pouch or marsupium, where the process is com- pleted. In the remaining Mammalia intra-uterine develop- ment goes much farther, the nutrition of the embryo being. in absence of the abundant food yolk of lower forms, effected by the aid of a placenta analogous but not homolo- gous to that of Mustelus, since developed, not from the yolk- sac, but from the allantois (see ANATOMY and MAMMALIA). The physiological processes of birth show a similar rise in complexity, due chiefly to the increasing strain upon the parental organization which this progress in the nutrition and protection of the embryo during its develop- ment involves ; for, while an ovum can be extruded by simple ciliary action, or at most by the gentle contractions of the oviduct, the expulsion of the relatively enormous Mammalian foetus involves mechanical difficulties of the most serious kind. And, besides these stresses and strains upon the pelvic basin itself or the muscular and connective tissues of the uterus, vagina, and its outlet, the inevitable rending asunder of the large closely interwoven and highly vascular placenta must evidently occasion an additional physiological disturbance. (4) Parental Care. Not to mention cases of mere conceal- ment of the ova or construction of egg cases, the lowest forms exhibiting such parental care are probably certain Holothurians and Starfishes described by Sir Wyville Thomson during the voyage of the " Challenger," in which the developing young are borne upon the dorsal surface of the parent. Many Crustaceans carry about their ova during development, and an Amphipod has been described as fol- lowed by its newly-hatched young like a hen by its chickens. The female Spider too, though ferocious towards the male, frequently spins a nest and shows some maternal solicitude; but such cases are far commoner among even the lower Vertebrates than the highest Invertebrates. Thus among Fishes the case of the nest-building Stickleback is especially familiar ; some Siluroids and Lophobranchs (and usually the males) carry about their young, the latter in ventral pouches, the former in the mouth. But the quaintest examples of care of offspring are those