284 REDUCTION OF THE CHROMOSOMES 



difference is clearly due to the fact that in the latter case the chromo- 

 somes are single or univalent, while in the former they are bivalent 

 (actually arising from dyads or double chromosomes). The remark- 

 able feature, on which too much emphasis cannot be laid, is that the 

 numerical difference should persist despite the fact that the mass, and, 

 as far as we can see, the quality, of the chromatin is the same in both 

 cases. In this fact we must recognize a strong support, not only of 

 Hacker's and Vom Rath's conception of bivalent chromosomes, but 

 also of the more general hypothesis of the individuality of chromo- 

 somes (Chapter VI.). 



i . Accessory Cells of the Testis 



It is necessary to touch here on the nature of the so-called " Sertoli-cells.," or sup- 

 porting cells of the testis in mammals, partly because of the theoretical significance 

 attached to them by M mot, partly because of their relations to the question of amito- 

 sis in the testis. In the seminiferous tubules of the mammalian testis, the parent- 

 cells of the spermatozoa develop from the periphery inwards toward 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, con- 

 taining large nuclei. The latter are the Sertoli-cells, 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 sperma- 

 tids, though their precise relationship is, to some extent, in doubt. The innermost 

 of these cells, next the iumen, 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 sper- 

 matogonia 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 r 

 and the same view was afterward especially advocated by Biondi ('85) and by Minot 

 ('92), the latter of whom regarded 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 ('70? ^e 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 CSS), von Ebner himself abandoned his early 

 hypothesis and strongly advocated Benda's views, adding the very significant result 

 that/w/r spermatids arise from each spermatocyle, precisely as was afterward shown 

 to be the case in Ascaris, etc. The very careful and thorough work of Benda and 

 von Ebner, confirmed by that of Lenhossek (""98, 2), leaves no doubt that mamma- 

 lian spermatogenesis conforms, in its main outlines, with that of Ascaris, the sala- 

 mander, and other forms, and that Biondi's account is untenable. Minors theoretical 

 interpretation of the Sertoli-cell, as the physiological equivalent of the polar bodies,, 

 therefore collapses. 



