258 FIELD. [Vol. XI. 



centrosome has been crowded out of the socket in the anterior 

 end of the nucleus (Fig. 20). In certain preparations which 

 I made of spermatozoa, killed in osmic vapor, stained in 

 Delafeld's haematoxylin, and mounted in dilute glycerine, 

 after a time the nuclei burst ; a portion of the darkly 

 stained nuclear contents escaped, leaving the cell-membrane 

 distinctly visible, I regard this membrane as the original cell 

 membrane which has persisted from the spermatid. Pictet 

 (18) thinks that this membrane dissolves and with the cyto- 

 plasm contributes to the formation of the tail, but in the case 

 of the Siphonophores he found that it persisted, but he adds 

 that " it is probable that it ultimately dissolves at the moment 

 of fecundation." 



Conclusion. 



Many of the facts found in this study of Echinoderm 

 spermatogenesis range themselves with a large mass of facts 

 which are being accumulated from all branches not alone 

 of the animal, but also of the vegetable realm, tending still 

 more to strengthen the theory first advanced by E. L. Mark, 

 and lately confirmed by O. Hertwig (32) and others that 

 the polar bodies are aborted eggs ; that there is a close par- 

 allelism between the histories of the cells concerned in the 

 formation of the egg and of the spermatozoon ; that the o.g'g, 

 the spermatozo5n, and the three polar bodies are strictly homol- 

 ogous ; and that any difference apparent is to be regarded as 

 a specialization for specific purposes ; the accumulation of all 

 of the food yolk in one of the ova results in the uselessness of 

 the other three (the polar bodies) ; the modification of the four 

 spermatids, derived from the spermatogone (which is the 

 homologue of the unmaturated ovum) by differentiation of the 

 cytoplasm into a vibratile tail, the separation and subsequent 

 extrusion of the no longer useful material of the nuclear 

 spindle, in the form of the mitosome, are modifications merely 

 of parts of the cell. (It is hardly necessary to call attention to 

 the absolute absence of homology between the extrusion of the 

 "polar bodies" and the extrusion of the "mitosome.") 



