Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/430

412 REPRODUCTION [ANIMAL. period of investigation, despite a few observations by Wagner and others, practically opens with Kbllikcr's fundamental observation (1841) that the head of each spermatozoon arose from the meta- morphosis of a seminal cell. In 1844 Haeckel described in Helix the cells destined to become spermatozoa as arising on the surface of an epithelial cell of the germ follicle. In a more elaborate paper (Z. W. Z., 1856) Kb'lliker compared the process of spermatogenesis in the Bull, Dog, and Rabbit with considerable detail, distinguishing in all cases the lining of the tubule into two kinds of cells, the outer having large nuclei and nucleoli, and undergoing rapid multiplication, while the thick inner layer of smaller cells was becoming differentiated into the true sperm-cells, which might be either unicellular or multicellular "cysts." "He also described the origin of the head of the sperma- tozoon (and indeed also that of its tail) from the nucleus of this parent cell, and the breaking loose of the spermatozoon from its parent. Henle (1865) showed that the tail was developed from a portion of the protoplasm, thus preparing for the comparison with ciliated epithelial .cells soon afterwards instituted by Schweigger-Seidel. Sertoli (1865) described certain " ramified cells" in the seminal tubules, and Merkel (1871) regarded these as forming a framework (Stutzzelleri) for other cells from which the spermatozoa developed. In the same year, however, appeared the important researches of Von Ebner, whose views will be understood by the aid of fig. 4. He described the large cells just mentioned as provided with a large and well-defined nucleus and nucleolus, and as being con- fluent at their base with each other, so forming a protoplasmic layer (Keimnetz), but stretching forwards into the lumen of the duct as irregular prolongations ; these become lobed or fingered, each lobe independently developing a nucleus, the large primitive nucleus remaining unchanged. To the whole prolongation with its lobes he applies the term spermatoblast. From each lobe a spermatozoon develops, the nucleus lengthening to form the main portion of the head, while a thin film of the protoplasm elongates into the cilium or tail. The young spermatozoa at first press I. III. IV. II. FIG. 4. Spermatogenesis (after Landois). I., cross section through a seminal tubule; II., unripe spermatoblast, with blunt rounded lappets, the young sperms; III., spermatoblast with ripe ciliated heads; IV., spermatoblast after separation of sperms. downwards into the body of the spermatoblast, so as to form bundles, but, when ultimately liberated, curl themselves up to roll down the seminal tubule, the more or less branched stump of the spennatoblast alone remaining along with the smaller cells of the tubules, which, according to him, take no part in the process, but merely perform nutritive or mechanical functions. A vigorous controversy at once arose, but Merkel claimed these spermatoblasts as being simply his " Stiitzzellen," and described the spermatozoa as arising from the small round cells, and as being only secondarily received into cavities of the former, a view which Henle also adopted. Sertoli also regarded Ebner's spermato- blasts as his "ramified cells," and ascribed all spermatogenetic functions to the round cells, which he distinguished in their youngest state as "germinative," and later as "seminiferous" cells, which then divided to form " nematoblasts, " these directly developing into spermatozoa. Blumberg (1873) attempted to reconcile the disputants by ascribing spermatogenetic functions both to the spermatoblasts and to the round cells. Neumann, while supporting Von Ebner in the main, describes his spermato- blasts as not processes of the nutritive network ("Keimnetz"), but as modified from the ordinary epithelium of the seminal tubule, and disputed the existence of Merkel's framework altogether ; and this essential confirmation of Von Ebner's view was supported by Mihalkovicz, Landois, and others. In a later paper he attempted to show that the lobes of Von Ebner's spermatoblasts might become separated off as the small round cells, and might then either break up or develop into spermatozoa. Krause (1876), while otherwise supporting Von Ebner, graphically described the sperma- toblasts as ciliated cells with ramified and even anastomosing processes, so doing away with any connective-tissue system altogether. Sertoli, however, continued to support his own view with greater elaborateness than ever. Semper's well-known researches (1875) on the urinogenital system of Elasmobranchs included an important contribution on spermatogenesis. He described an invagination of the germinal epithelium or of the primitive ova into the subjacent stroma, win-re they form a primitive follicle, which again comes into relation with the incipient tubule. The central cell of the follicle undergoes mucous degeneration and becomes absorbed, leaving a cavity lined by a single layer of epithelial cells, which divide rapidly, becoming cylindrical, with large round nuclei. These divide into a " cover- cell" and a "mother-cell"; the nucleus of the latter divides repeatedly until about sixty sperm nuclei are formed, which elongate to form the heads of the spermatozoa. As these develop they come to lie in a bunch, lying into the cavity of the follicle, their expulsion being effected by the swelling of the nucleus of the cover-cell. An important series of researches by Von La Valette St George on spermatogenesis among Vertebrates was meanwhile in progress. In his final paper (1878) he describes the seminal tubules as con- taining two distinct types of cell. One kind, resembling young ovules, which he terms primitive seminal cells or spermatogonia, divide into a mass or spennalogcmma of small cells (sperma- tocytcs). These spermatocytes may either () all develop into spermatozoa (Mammals), or (b) a single spermatocyte may become modified as a basilar cell (Plagiostome Fishes), or (c) a number may form an envelope or cyst around the qthers (Amphibians and Fishes). The second kind of primitive cells he terms "follicle- cells," and regards as non-essential ; these are united into a loose tissue and envelop the spermatogonia and spermatogemma. In this view we have a marked divergence from both preceding theories, as the author does not hesitate to point out. He regards Von Ebner's "Keimnetz" and " spermatoblasts " as confounding both his follicle cells with their included spermatogemmce, and interprets Neumann's figures confirmatory of Von Ebner in the same way. Merkel's " Stiitzzellen " he identifies as his " follicle- cells " altered by reagents. The subsequent researches of Helman (1880), Krause, and Nussbaum among Vertebrates need only be mentioned as essenti- ally confirmatory of the observations of La Valette. Meyer's careful observations (1880) also led him to the same general view, with the important difference, suggested by one of the preceding authors, that the " follicle cells " of La Valette were not inert as he supposed, but the earliest stages of his spermatogonia and spermatogcmmce. Klein's views (1881) have perhaps most relation to those of Sertoli. Some important work was meanwhile being done among Inverte- brates. In 1877 F. E. Schultze described spermatogenesis in the Sponge Halisarca,a. germinal cell, analogous to that which gives rise to an ovum, becoming covered by an epithelium, and divid- ing into a multitude of segments, each of which becomes drawn out into a spermatozoon. In 1844 Meckel had described the sper- matozoa of the Snail as arising superficially from a mother-cell, and this view had been confirmed by various authors. In 1879 Duval described in the same animal the spermatocytes as arising by the endogenous division of a mother-cell, and subsequently coming to its surface, and Halle/ described an essentially similar process in some Flanarians. Graff, however, found in other species that no remnant survived, but that the whole of the spermatogonia became converted into spermatocytes. In 1880 Blomfield investi- gated the process in the Earthworm, his results resembling rather those of Von Ebner (fig. 4) than those of La Valette. Strongly em- phasizing, however, the importance of the nutritive basal cell or spermatoblast of Von Ebner, he proposed yet another nomenclature. In a subsequent paper (1882) he describes the process in the Snail and the Frog. The former agrees substantially with the Worm, but in the Frog a hollow spermatogemma arises; each of its cells elongates to form a spermatozoon ; these while still immature arrange them- selves in bundles round one of the more superficial cells, which " become blastophoral corpuscles," a view which recalls Merkel's explanation of Von Ebner's spermatoblasts, mentioned above, viz., that the spermatozoa only became temporarily lodged in their recesses after completing their development. In this respect the view of Renson (1882) is especially interesting. He describes the follicular cells of La Valette as segmenting into FIG. 5 (after Renson.)!, folliculur cells ; 2, spermatogemma ; 3-8, separate ncmatoblasts developing into spermatozoa ; 9, nematoblasts grouped on sup- porting cell ; 10-11, successive stages of the penetration of the young sper- matozoa. " multinuclear cysts" (spermatogemmse), of which the constituent " nematoblasts " develop into young spermatozoa. These immature