XIV 



YOLK AND YOLK CELLS 



In the chapter on "Cleavage" it is stated that not all of the 

 cleavage cells migrate to the periphery of the egg, a few of these 

 remaining behind in the yolk to form the yolk cells. In order 

 to distinguish them from cells entering the yolk at a later period 

 these yolk cells will be referred to as primary yolk cells, since 

 they are primary in respect to the time of their origin. At first 

 the primary yolk cells preserve the appearance of ordinary cleav- 

 age cells and divide simultaneously with their sister cells, which 

 are about to form the blastoderm, exhibiting mitotic figures 

 precisely like those of the cleavage cells (Fig. 93, A and B). 

 Soon after the blastoderm has become established mitotic divi- 

 sions of this character cease and are supplanted by mitoses of a 

 more or less irregular character. In these the spindles are as 

 a rule short and the chromosomes frequently agglomerated to 

 form irregular masses (Fig. 93, C-G). This period lasts until 

 after the nuclei forming the blastoderm have become arranged 

 in two layers (18-20 hrs.). The large number of cells found 

 in the yolk at this time is therefore to be attributed to mitotic 

 divisions, since the latter are found abundantly until some time 

 after the blastoderm has been formed. This includes both the 

 earlier regular type as well as the later irregular type. Many 

 of the mitotic figures of the later type resemble those found in 

 pathological tissues such as cancer cells and often have the 

 appearance of being unequal (Fig. 93D), or multipolar (Fig. 

 93, F and G), but the mitotic figures are so minute and the sur- 

 rounding cytoplasm often so dark that a critical study of them 

 was not attempted. The probability that both the unequal and 

 multipolar types exist is strengthened by the subsequent frequent 

 presence of nuclei of different sizes within the same mass of 

 crytoplasm (Fig. 92A). '[These irregular figures might be con- 

 sidered actually pathological or abnormal, were it not for their 

 great abundance in several different preparations of eggs of the 



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