74 



CYTOLOGY 



CHAP. 



merely to the fact that the chromatin is more concentrated in the former, 

 for as it approaches the female nucleus it grows larger, and at the same 

 time becomes looser in texture, till ultimately the two gamete nuclei 

 are generally of the same volume and structure. 



Since each gamete nucleus is haploid, it is obvious that the zygote 

 nucleus is diploid again. 



As we have seen, a typical spermatozoon consists of three main 

 portions— head, middle piece, and tail. The fate of the head or nucleus 



Fig. 33. 



Syngamy in Chaetopterus pergamenlaceus. (After Mead, J.M., 1898.) A, s6on after entry of spermato- 

 zoon ; 9 nucleus in anaplxase I. ; S aster developing ; B, 9 nucleus in anaphase II. ; C, 9 nucleus in form 

 of karyomeres (see p. 131) ; 9 achromatic figure has disappeared ; D, approach ; E, fusion, of gamete nuclei. 



P.b.i and P.h.2, first and second polar bodies. 



we have already followed. The tail apparently takes no part in fertiliza- 

 tion. (For the fate of the chondriosomes, which are usually situated in 

 the tail, see p. 198.) Sometimes it does not even enter the Qgg, being 

 broken off from the middle piece as soon as this has penetrated into the 

 egg (sea-urchins). When the tail does enter the egg it breaks off from 

 the rest of spermatozoon soon after entry, and lies in the egg cj^toplasm 

 for a time, and then degenerates. 



There remains to be considered the middle piece, with which the 

 centrosome, as the development of the spermatozoon showed us, is 

 generally related. Owing to the mode of entr}^ of the spermatozoon — 

 head first — the middle piece is at first behind the head. After entry, 

 however, the head and middle piece rotate so that the latter is in front 

 of the former during its journey towards the centre of the egg. The 



