Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/431

ANIMAL.] REPRODUCTION 413 nematoblasts group themselves round the extremity of certain long projecting epithelial cells, the supporting cells (or "cellules de soutien," obviously the spermatoblasts of Von Ebner), and actually sink into their protoplasm to complete their development. When fully developed, the heads of the young spermatozoa have attained the base of the supporting cell, but this now elongates and bears them anew to the lumen of the duct, where new nematoblasts are by this time waiting to take their place (fig. 5). The researches of Hermann (1882) in Elasmobranchs are broadly confirmatory of those of Semper, while those of Jensen and of Swaeu and Masquelin (1883) are especially corroborative of the views of La Valette. The latter are, however, of importance as tend- ing towards reconciliation. The primitive ampullae being formed, the " male ovules " and the follicular cells are henceforth distinct : the former segment into spermatogemmae ; the latter (at first form- ing incomplete envelopes to the male ovule) mostly disappear, save one which travels downwards until it lies between the wall of the ampulla and the spermatogemma, thus constituting its "basilar cell." The spermatogemma meanwhile is developing a central cavity ("loge caudale"), from which the tails of the incipient spermatozoa or "nematoblasts" project into the lumen of the duct. The basilar cell has also been enlarging, and fusing with the intercellular substance of the spermatogemma ; the nematoblasts thus come to be plunged into the basilar cell, and sink downwards towards its nucleus ; but this again elongates to expel them. In the Salamander these follicular cells form a complete envelope to the male ovules during their whole segmenta- tion and subsequent evolution. In Mammals the male ovule divides into an active and a temporarily inert portion (follicular cell of La Valette, germinative cell of Sertoli and Renson) ; the former segments into the spermatogemma, of which the resultant nematoblasts plunge into the basilar cells, much as described by Renson. They compare the intercellular substance of the sper- matogemma to the blastophor described by Blomfield in Worms, and regard the follicular cells as a secondary addition peculiar to Vertebrates, and homologous with the follicular ( cells of their ova. The first step towards any understanding of the process of spermatogenesis amid this maze of controversy is to collate the various observations ; hence the present attempt to summarize the main observations on the subject, and afford a key to the nomencla- ture. But how shall we reconcile the different theories ? Each author formulates his own view of spermatogenesis, and sometimes even admits only a single method (e.g., Von Ebner, Blomfield, &c.); yet, unless we attach considerably greater weight to the observations of at least a majority of all these workers than they sometimes incline to grant to those of each other, the literature and iconography of histology become of little worth. Since La Valette, however, most observers have admitted the existence of several methods ; FIG. 6. Hypothetical comparison of oogenesis and spermatogenesis. The forms the homology indeed the primordial identity of the primitive germinal cells of male ovule with female ovule has been often pointed out, and the general resemblance of the process of spermatogenesis to that of segmentation has been noted, and even accented by the use of terms like sperm-morula, sperm-blastula, &c. But it is not enough to make such a comparison in general terms ; it must be either susceptible of definite refutation or con- firmation by appeal to details ; and the present writer has hence elsewhere attempted to compare the various modes of spermato- genesis with the corresponding modes of segmentation of the ovum, and so reconcile the conflict of testimony and opinion upon the subject. (8) Ontogeny of the Spermatozoon. The origin of the spermatozoon from the spermatocyte is, as we have seen above, a phenomenon of considerable complexity. It has long been known to arise from the nucleus and the tail from the protoplasm, and Flemming in 1880 gave further precision to our knowledge by tracing the head of the spermatozoon of Salamandra from the chromatiu of the nucleus. Various observers had also noted, besides the nucleus, the appear- ance of a small denser mass of protoplasm within the spermatocyte, the " accessor}' corpuscle" (Nelcnkern, corpuscle precurseur), but its origin and fate have scarcely yet been settled with complete clear- ness. It appears, however, to arise from the nucleus, the remaining portion of the nucleus going to form the main portion of the head, while a film of superjacent protoplasm stretches over it, thickens somewhat to form the middle piece, and becomes drawn out into the filamentous tail, which frees itself from the remains of its sper- matocyte and swims away (3 to 8 in fig. 5). Von Brunn has recently described in Birds the origin of this accessory corpuscle with especial clearness by the division of the nucleus of the developing spermato- zoon. In Mammalia the " cap " of the young spermatozoon is de- scribed by Renson as nucleus, and by others as protoplasm, and it has been compared by many authors to the polar vesicle of the ovum. In Plagiostome Fishes Semper describes, and others con- firm, the existence of an additional nucleus, or "problematic body," which appears to correspond to the sum of the accessory corpuscles of all the spermatozoa, and this, after their escape, has been shown by Swaen and Masquelin to fuse with the nucleus of the basilar cell. The resemblance of this embryonic and transi- tory structure to the permanent post-nuclear segment of the curious almost amoeboid spermatozoon of Ascaris is closely sugges- tive of their homology, and it is also interesting to note, in figures of the developing spermatozoa of Selachians, their considerable resemblance to the fringed adult spermatozoa of many Platyhel- minthcs. (8) The Ovum. History. From the earliest times naturalists have of course been familiar with the form and function of the ova of a great number of animals ; the only seriously disput- able question (excepting that of spontaneous generation) has been that of the like origin of the embryo in Mammals. The history of the discovery is a curious one. Galen had described the human ovaries as testes muliebres, but the term ovary is due to Steno (1664), who like our most modern investigators started in his comparisons from the corresponding organs in Sharks and Eays. In 1672 Regner de Graaf, in a remarkable work upon the structure of the ovary and its accessory organs, not only described in the ovary in Birds and Mammals the follicles which now bear his name, and which he regarded as the ova, but made the generalization of the universal occurrence of ova through- out the animal kingdom, and even observed the ovum in the oviduct of the Rabbit. His opinion, however, was overborne by the authority of his more famous countryman Leeuwenhoek, who regarded not the oviduct but the ovary, and more precisely the corpus luteum, as the seat of de- velopment of the embryo. The observation of De Graaf was repeated at the end of last century by Cruikshank, and again by PreVost and Dumas. The definite establishment of our present knowledge of the Mammalian ovum dates only from 1827, when Von Baer clearly traced the ovum from the uterus back to its earliest appearance in the ovi- duct, and thence to its origin within the Graafian follicle. Piirkinje had meanwhile (1825) described the "germinal vesicle " in the Chick, Coste showed its occurrence in the Mammalian ovum, and Wagner discovered (1836) the " germinal spot " in Mammalian and other ova. (9) Mass and Chemical Composition of the Ovum. The absence of any extensive measurements of the ovum in the various groups, either absolutely or in comparison with the bulk of the parent, renders it impossible to generalize with any great degree of definiteness. While from some comparisons it is at once apparent that the higher the organization the larger the ovum, many inde- pendently variable factors affect this, e.g., the size, organi- zation, or maturity of the parent, the duration of stay in the oviduct, and the climate and other surroundings. Examples of such variation are readily seen ; it is natural that self-supporting larval forms should start with smaller food-yolk than those which develop before leaving the egg, and among placental Mammals that the continuous supply of nutriment from the parental resources should