208 REDUCTION OF THE CHROMOSOMES 



APPENDIX 



I . Accessory Cells of the Testis 



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

 supporting cells of the testis in mammals, partly because of the theoretical signifi- 

 cance attached to them by iMinot, 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 

 lum.en, 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 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. Tlie 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 sperma- 

 tids, 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 physio- 

 logical 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 spermato- 

 genesis 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 tlie 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 iMinofs, wide 

 of the mark, and they were advanced before the real parallel between spermato- 

 genesis and ovogenesis had been made known by Platner and Hertwig. 



