84 CYTOLOGY 



CHAP. 



By the time that the i6-cell stage is reached this lagging has gone 

 so far that, when the remainder of the cells divide to form what 

 should be the 32-cell stage, the granule cell fails to divide, at all, so that 

 the blastula at this stage contains only thirty-one cells. 



After the fourth division of the granule cell — i.e. in the 16- and 

 31-cell stages — the granules are no longer confined to one attraction 

 sphere, but are scattered throughout the whole cell. Consequently, when 

 the granule cell divides next time, which it does at the close of the 

 31-cell stage, it produces two similar granule cells. These cells, which 

 remain without further division for a considerable time, are the primitive 

 germ -cells, from which the right and left gonads develop respectively. 



Thus the germ-track in Cyclops and the allied genera is quite as clearl}^ 

 marked out as in Ascaris. At first sight it might, however, appear that 

 the two processes were very distinct, the one being a case of nuclear, and 

 the other of c^^toplasmic, differentiation. Nevertheless, while we do not 

 know the significance of the diminution of the chromatin and fragmenta- 

 tion of the chromosomes in the somatic cells of Ascaris, there is little 

 doubt that in both cases it is the nature of the cytoplasm of the cell which 

 determines whether it shall be a somatic or a gonadic cell. This is 

 fairly plain in the case of Cyclops, where it is clear enough that the 

 granules are of cytoplasmic origin. Amma gives reasons for the belief 

 that they are temporary metabolic products of a special portion of the 

 cytoplasm. We may conceive of this special cytoplasm as concentrated 

 in the undivided egg near one of the centrosomes, and consequently 

 passing into only one of the daughter cells, until, after four such divisions, 

 the cells have become so much reduced in size that now this substance 

 occupies the whole, or nearly the whole, of the cell instead of one pole 

 only of it. Henceforth division of this cell, or of its descendants, must 

 result in the passage of this substance into both 'daughter cells. 



By the study of the diminution process in dispermic Ascaris eggs, 

 Boveri (1910) has shown that in this species also it is the nature of the 

 cytoplasm which determines whether diminution shall or shall not take 

 place in a particular cell. The eggs in question are the very rare abnormal 

 cases where two spermatozoa have entered the egg. Both centrosomes 

 introduced by the spermatozoa divide, and then form a quadripolar 

 spindle figure (cf. Fig. 74). On this spindle the 3^, or 6, chromosomes 

 derived from the female and the two male gamete nuclei take up their 

 position (the account deals with A. m. bivalens). As a rule the first 

 cleavage mitosis of such an egg divides it simultaneously into four 

 blastomeres, and the twelve daughter chromosomes of the original six 

 are distributed among the four nuclei in a most irregular manner ; for 

 example, one nucleus may get only one chromosome, another three, and 

 the other two four each, etc. 



