MONOGRAPH OF THE EXISTING CRINOIDS. 363 



which composed the spindle fibers are seen scattered through the cytoplasm; the 

 nuclei themselves, rounded up and with a nuclear membrane, are free from the 

 granules. In the cytoplasm also, close beside each nucleus, remains one of the 

 centrosomes which took part in the division of the cell. 



According to Field, the probable history of the centrosomes is as follows: 

 The original centrosome appears first in the spermatogone and is of intranuclear 

 origin. Field can not say that the centrosome is not present in the spermatogones 

 while they are still close to the germinal epithelium, but he has never seen it 

 previous to the disappearance of the nucleolus, and when first seen it seems to be 

 within the nuclear membrane. 



In the spermatogone it divide? into two, which participate in the mitotic 

 division resulting in two spermatocytes. 



With each spermatocyte nucleus there is left one centrosome, which is one- 

 half of the original centrosome of the spermatogone. Initiatory to the mitotic 

 division of the spermatocytes, this centrosome divides into two which come to lie 

 at the poles of the spindle. With the completion of this mitosis each spermatid 

 contains one centrosome. Thus from the original centrosome of the spermatogone 

 four centrosomes have been derived, and each of these four centrosomes comes to 

 be placed at the apex of the head of a spermatozoon, and in the fecundation 

 process enters the o^iim with the nucleus of the spermatozoon and takes part in 

 the subsequent fertilization. 



"\ATien the nuclear membrane is formed ai'ound the nucleus of the spermatid 

 the centrosome is not included, but is left outside in the cytoplasm. This fact 

 either points to the probability that the centrosome is extranuclear in the spermato- 

 gone, or else that for some reason the condition in the spermatogone varies from 

 that in the spermatid. If the latter is true, a possible explanation for this differ- 

 ence may be found in the fact that with the spermatid the series of mitotic divisions 

 is completed, and that subsequently the constituent parts of the cell will undergo 

 an extreme modification. 



With the formation of the tail it first begins to be possible to say which part 

 of the spermatid will be the anterior point of the spermatozoon. The centrosome, 

 which hitherto apparently has had no special position with reference to the nucleus, 

 now comes to lie close beside it, and usually directly opposite the point where the 

 tail is forming. With the diminution of the cytoplasm around the nucleus the 

 centrosome comes to lie in a depression in the nucleus, but remains entirely outside 

 of the nuclear membrane. This condition is probably brought about mechanically 

 by the pressure of the tightly drawn cell membrane which invests the spermatozoon, 

 pushing the unyielding refringent centrosome into the wall of the nucleus. 



The sperm centrosome is the centrosome of the spermatid and of the previous 

 cell generations, and it continues in company with the nucleus of the spermatozoon 

 until the completion of the process of fertilization. 



As soon as it has passed the denser outer layer of the cytoplasm of the egg 

 it draws farther away from the nucleus and the characteristic radiations appear 

 in the cvtoplasm. It no longer directly precedes the nucleus, but comes to lie nt 

 one side. 



