Fig. 384—A, Part of the kidneys of Bdellostoma. a prorenal duct (nephroductus), b segmental or primitive urinary canals (pronephridia), c renal or Malpighian capsules. B Portion of same, highly magnified. c renal capsules with the glomerulus, d afferent artery, e efferent artery. From Johannes Müller (Myxinoides).
The sexually-mature Amphioxus is not hermaphroditic, as its nearest invertebrate relatives, the Tunicates, are, and as the long-extinct pre-Silurian Primitive Vertebrate (Prospondylus, Figs. 98–102) probably was. The actual lancelet has gonochoristic structures of a very interesting kind. As we saw in the anatomy of the Amphioxus, we find the ovaries of the female and the spermaries of the male in the shape of twenty to thirty pairs of elliptical or roundish four-cornered sacs, which lie on either side of the gut on the parietal surface of the respiratory pore (Fig. 219 g). According to the important discovery of Rückert (1888), the sexual glands of the earliest fishes, the Selachii, are similarly arranged. They only unite afterwards to form a pair of simple gonads. These have been transmitted by heredity to all the rest of the Craniotes. In every case they lie originally on each side of the mesentery, underneath the chorda, at the bottom of the body-cavity. The first traces of them are found in the cœlom-epithelium, at the spot where the skin-fibre layer and gut-fibre layer meet in the middle of the mesenteric plate (Fig. 93 mp). At this point we observe at an early stage in all craniote embryos a small string-like cluster of cells, which we may call, with Waldeyer, the “germ epithelium,” or (in harmony with the other plate-shaped rudimentary organs) the sexual plate (Fig. 173 g). This germinal or sexual plate is found in the fifth week in the human embryo, in the shape of a couple of long whitish streaks, on the inner side of the primitive kidneys (Fig. 183 t). The cells of this sexual plate are distinguished by their cylindrical form and chemical composition from the rest of the cœlom-cells; they have a different purport from the flat cells which line the rest of the body-cavity. As the germ epithelium of the sexual plate becomes thicker, and supporting tissue grows into it from the mesoderm, it becomes a rudimentary sexual gland. This ventral gonad then develops into the ovary in the female Craniotes, and the testicles in the male.
In the formation of the gonidia or erotic sexual cells and their conjunction at fecundation we have the sole essential features of sexual reproduction; but in the great majority of animals we find other organs taking part in it. The chief of these secondary sexual organs are the gonoducts, which serve to convey the mature sexual cells out of the body, and the copulative organs, which bring the fecundating male sperm into touch with the ovum-bearing female. The latter organs are, as a rule, only found in the higher animals, and are much less widely distributed than the gonoducts. But these also are secondary formations, and are wanting in many animals of the lower groups.
In the lower animals the mature sexual cells are generally ejected directly from the body. Sometimes they pass out immediately through the skin (Hydra and many hydroids); sometimes they fall into the gastric cavity, and are evacuated by the mouth (gastræads, sponges, many medusæ, and corals); sometimes they fall into the body-cavity, and are ejected by a special pore (porus genitalis) in the ventral wall. The latter procedure is found in many of the worms, and also in the lowest Vertebrates. Amphioxus has the peculiar feature that the mature sexual products fall first into the mantle-cavity; from there they are either evacuated by the respiratory pore, or else they pass through the gill-clefts into the branchial gut, and so out by the mouth (p. 185). In the Cyclostomes they fall into the body-cavity, and are ejected by a genital pore in its wall; so also in some of the fishes. From these we gather the features of our earlier ancestors in this respect. On the other hand, in all the higher and most of the lower Vertebrates (and most of the higher Invertebrates) we find in both sexes special tubular passages of the sexual gland, which are called “gonoducts.” In the female they conduct the ova from the ovary, and so are called “oviducts,” or “Fallopian tubes.” In the male they convey the spermatozoa from the testicles, and are called “spermaducts,” or vasa deferentia.
Fig. 385—Transverse section of the embryonic shield of a chick, forty-two hours old. (From Kölliker.) mr medullary tube, ch chorda, h horny plate (skin-sense layer), ung nephroduct, vw episomites (dorsal primitive segments), hp skin-fibre layer (parietal layer of the hyposomites), dfp gut-fibre layer (visceral layer of hyposomites), ao aorta, g vessels. (Cf. transverse section of duck-embryo, Fig. 152.)
The original and genetic relation of these two kinds of ducts is just the same in man as in the rest of the higher Vertebrates, and quite different from what we find in most of the Invertebrates. In the latter, as a rule, the gonoducts develop directly from the embryonic glands or from the outer skin; but in the Vertebrates an independent organic system is employed to convey the sexual products, and this had originally a totally different function—namely, the system of urinary organs. These organs have primarily the sole duty of removing unusable matter from the body in a fluid form. Their liquid excretory product, the urine, is either evacuated directly through the skin or through the last section of the gut. It is only at a later stage that the tubular urinary passages also convey the sexual products from the body. In this way they become “urogenital ducts.” This remarkable secondary conjunction of the urinary and sexual organs into a common urogenital system is very characteristic of the Gnathostomes, the six higher classes of Vertebrates. It is wanting in the lower classes. In order to appreciate it fully, we must give a comparative glance at the structure of the urinary organs.