Maturation of Parthenogenetic Eggs

The first view must obviously stand or fall with the conception of the primary chromatin-rods as bivalent chromosomes. That this is a valid conception is in my judgment demonstrated by Brauer's remarkable observations on for in this case it is impossible to escape the conclusion that the " chromosomes " of those parthenogenetic embryos in which the number is halved are bivalent, — i.e. have the value of two chromosomes united by their ends, and they lend the strongest support to vom Rath's and Hacker's hypothesis. For if the number of chromosomes be merely the expression of a formative tendency, like the power of crystallization, inherent in each specific kind of chromatin, why should the chromatin of the same animal differ in the two cases though derived from the same source in both ? Yet if the cleavage-nucleus arises from eighty-four dyads the same number of chromatin-rods appears in all later stages ; whereas if the dyads break each into two separate chromosomes before their union, the number is thenceforward one hundred and sixty-eight. So great is the force of this evidence that I think we must still hesitate to accept the results thus far attained in Ascaris and the plants, and must await further research in this direction. Until the contradiction is cleared up the problem of reduction remains unsolved.

Accessory Cells of the Testis

It is necessary to touch here on the nature of the so-called "Sertoli-cells," or supporting cells of the testis in mammals, partly because of the theoretical significance attached to them by Minot, partly because of their relations to the question of amitosis in the testis. In the seminiferous tubules of the mammalian testis, the parent-cells of the spermatozoa develop from the periphery inwards towards the lumen, where the spermatozoa are finally formed and set free. At the periphery is a layer of cells next the basement-membrane, having flat, oval nuclei. Within this, the cells are arranged in columns alternating more or less regularly with long, clear cells, containing large nuclei. The latter are the or supporting cells ; they extend nearly through from the basement-membrane to the lumen, and to their inner ends the young spermatozoa are attached by their heads, and there complete their growth. The spermatozoa are developed from cells which lie in columns between the Sertoli-cells, and which undoubtedly represent spermatogonia. spermatocytes, and spermatids, though their precise relationship is, to some extent, in doubt. The innermost of these cells, next the lumen, are spermatids, which, after their formation, are found attached to the Sertoli-cells, and are there converted into spermatozoa without further division. The deeper cells from which they arise are spermatocytes, and the spermatogonia lie deeper still, being probably represented by the large, rounded cells.

Two entirely different interpretations of the Sertoli-cells were advanced as long ago as 1871, and both views still have their adherents. Von Ebner ('71) at first regarded the Sertoli-cell as the parent-cell of the group of spermatozoa attached to it, and the same view was afterwards especially advocated by Biondi ('85), and is still maintained by Minot ('92), who regards the nucleus of the Sertoli-cell as the physiological analogue of the polar bodies, i.e. as containing the female nuclear substance

('92, p. 77). According to the opposing view, first suggested by Merkel ('71), the Sertoli-cell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell, which serves merely as a support and a means of conveying nourishment to the growing spermatozoa. This view was advocated by Brown ('85), and especially by Benda ('87). In the following year ('88), von Ebner himself abandoned his early hypothesis and strongly advocated Benda's views, adding the very significant result that four spermatids arise from each spermatocyte, precisely as was afterwards shown to be the case in Ascaris, etc. The very careful and thorough work of Benda and von Ebner leaves no doubt, in my opinion, that mammalian spermatogenesis conforms, in its main outlines, with that of Ascaris, the salamander, and other forms, and that Biondi's views, which Minot unfortunately adopts. are without foundation. If this be the case. Minot's theoretical interpretation of the Sertoli-cell as the physiological equivalent of the polar bodies, of course collapses.

Various other attempts have been made to discover in the spermatogenesis a casting out of material which might be compared with the polar bodies, but these attempts have now only an historical interest. Van Beneden and Julin sought such material in the " residual corpuscles" left behind in the division of the sperm-forming cells of Ascaris. Other authors have regarded in the same light the " Nebenkern (Waldeyer) and the "residual globules" (Lankester, Brown) thrown off by the developing spermatozoa of mammals. All of these views are, like Minot's, wide of the mark, and they were advanced before the real parallel between spermatogenesis and ovogenesis had been made known by Platner and Hertwig.

Amitosis in the Early Sex-Cells

Whether the progenitors of the germ-cells ever divide amitotically is a question of high theoretical interest. Numerous observers have described amitotic division in testis-cells, and a few also in those of the ovary. The recent observations of Meves ('91), vom Rath ('93), and Preusse ('95), leave no doubt whatever that such divisions occur in the testis of many animals. Vom Rath, however, maintains, after an extended investigation, that all cells so dividing do not belong in the cycle of development of the germ-cells ('93, p. 164) ; that amitosis occurs only in the supporting or nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate, like the residual bodies" of Van Beneden. Meves has, however, produced strong evidence ('94) that in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in the ensuing spring may again resume the process of mitotic division, and give rise to functional spermatozoa. On the strength of these observations, Flemming ('93) himself now admits the possibility that amitosis may form part of a normal cycle of development, and Preusse has recently shown that amitosis may continue through several generations in the early ovarian cells of Hemiptera without a sign of degeneration.