Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/427

Rh ANIMAL.] REPRODUCTION 409 derm of the nutritive polyp. These conclusions not only invalidate Van Beneden's attractive theory, but tend to overthrow the ordinary view of the alternation of genera- tions in the Hydromedusse. Vermes. The incipient ovary and testis are seen in their simplest expression in such a case as that of the hermaphrodite Bryozoa, or even better in the low Chseto- pod Worm Tomopteris (fig. 1), in which a patch of cells of the lining membrane of the coelom proliferate and enlarge into ova, or divide into sperma- tozoa, fall off, and become ex- truded. In higher Worms (e.g., Lumbricus) the glands become localized in definite segments, _ ,, , ' FIG. 1 (from Gegenbaur). Ova ongi- and excretory ducts, apparently natiug from the lining epithelium specialized segmental organs, ap- of a P'?""' of TomopuH,. pear. Among the Platyhelminthes, on the other hand, the most extraordinary specializations occur. The generative apparatus reaches a complexity which in some respects even excels that of the higher Vertebrates, and which, since it occurs not only among the parasitic forms but also in their less modified free-living allies (if not ancestors), the Turbellarians, must be regarded, not as any mere adapta- tion to parasitic life, but as an important factor in explaining (by the peculiar advantage which such increased reproductive efficiency affords ; see PARASITISM) the wide prevalence of parasitism in the Platyhelminthes. Echinodermata. Here the reproductive organs are of extreme simplicity mere lobed glands usually provided with a duct or pore, or sometimes merely bursting into the body-cavity. Arthropoda. The essential organs never exceed one pair, a simple median germ-gland being probably, as Gegenbaur suggests, the primitive condition. The acces- sory organs, however, reach great complexity, especially among the higher Insecta. Mollusca. The lower Lamellibranchs have paired herm- aphrodite glands opening in relation to the excretory organs ; in the hermaphrodite Gasteropoda, however, high complexity of the accessory organs occurs (e.g., Helix). In the dioecious Cephalopoda the oviduct is single, and there are remarkable accessory female glands, while in the male the formation of the spermatophores and the curious modi- fication of an arm for copulatory purposes are noteworthy. Tunicata, In this group (so frequently hermaphrodite) the reproductive apparatus is again greatly reduced, the paired or single sexual glands being sometimes even duct- less, while accessory organs are absent. Vertebrata. Starting again, in the lowest Vertebrates, with organs of an exceedingly simple and primitive kind, we find the series presenting all gradations up to a very high complexity. Thus in Amphioxus the reproductive glands are little modified patches of the lining epithelium of the ccelom, and arise in a paired series, an arrange- ment recalling that of the simpler segmented Worms. In other Vertebrates, however, the essential organs are most distinctly localized in origin, never exceeding a single pair, and are also much more evolved in structure. In its earliest recognizable state the generative gland is a slight thickening of the peritoneal epithelium and the subjacent connective tissue on each side of the mesentery, lying near and parallel to the incipient renal apparatus. Leaving the histological details of the process by which this "germinal epithelium," with the subjacent connective tissue of the " genital ridges," develops into ovary and testis with their characteristic products to the sections dealing with oogenesis and spermatogenesis respectively, and confining ourselves to the gross anatomy of these essential organs, we may note their very simple character in the Marsijwbranchii, where in the Lampreys they extend for a great length along the coelom, and exhibit a number of tolerably regular lamellar folds, recalling the segmental arrangement of lower forms. In the remain- ing Vertebrates these organs are usually less elongated and relatively smaller and more compact, though it is interesting to note that some Amphibians (notably many C&cilix) exhibit traces of a more or less discontinuous, perhaps serial, arrangement. In Plagiostome Fishes the sexual glands arise along only a portion of the genital ridge, the remainder having its stroma mainly enlarged, and forming the so-called epigonal gland. In many of the higher Amphibians accessory organs of unknown function, the so-called fatty bodies or corpora adiposa, are attached to the ovaries and testes ; and in the Toad and other Anura another organ in close relation to the testis, and in histological structure resembling a rudi- mentary ovary, has also been described. A considerable tendency towards loss of symmetry appears in the essential organs, particularly the ovaries, of many Vertebrates. Thus in Myxine neither ovary nor testis is present on the left side, and in many Sharks and Dogfish the left ovary is rudimentary. In Snakes the left ovary is smaller than the right and usually lies behind it, while in Birds the left ovary is alone functional, that of the right side becoming so completely atrophied at an early stage of development that traces of it in the adult are only found surviving in a few forms, especially some of the diurnal birds of prey. Among Mammals symmetrical development is the rule, yet in the curiously Bird-like Ornithorhynckus the left ovary is much smaller than the right. The relative size of the ovaries varies greatly throughout the Vertebrate series, in relation partly to the relative proportion of stroma to germinal tissue in the histological structure of the organ, to the fecundity of the species, and to the number of ova produced, partly also to the presence or absence of a food yolk, and the consequent size of the ova. In many forms a great increase of size takes place at breeding-time. In the majority of Vertebrates, as in lower forms, the essential organs remain throughout life in the position in which they develop, or at most depend into the ccelom supported by a mesenteric fold. In most Mammals, how- ever, a certain change of position takes place, the ovaries usually shifting more or less backwards towards the pelvis. The testes too in the Monotremata leave their embryonic position at the inner edge of the primitive kidneys and travel backwards. In Edentates, Hyrax, Elephants, and Cetaceans they remain near or a little below the kidneys, but they usually reach the abdominal wall, which they may more or less completely pass through in the inguinal region (as in many of the lower Eodents and Carnivores), or even descend into a more or less distinct diverticulum or hernial protrusion of the integuments of .the abdominal wall, the scrotum. This protrusion arises usually at the posterior margin of the primitive urinogenital opening, but, by exception, in Marsupials in front of it. The cavity of the scrotal pouch may remain throughout life in con- tinuity with that of the abdomen, so enabling the testes to pass backwards and forwards at each breeding season (Marsupials, Rodents, Insectivora, Bats, &c.), while in the higher forms, e.g., Ungulates, Primates, &c., the scrotum retains the testes permanently shut off from the abdominal cavity. The origin and homologies of the genital ducts in Vertebrates, and the accessory organs in relation to them, may now, after a very great amount of anatomical and embryological inquiry, be considered as tolerably settled, at least in their main outlines. XX. - 52