THE EMBRYOLOGY OF THE HONEY BEE 237 



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Fig. 95. Section of the blastoderm of an egg eighteen to twenty hours 

 old, showing nuclei leaving the blastoderm and entering the yolk, x 840. 



their formation. The immigration of secondary yolk cells from 

 the blastoderm into the yolk is most evident in preparations of 

 eggs eighteen to twenty hours old. It may be observed in eggs 

 as old as twenty- four to twenty-six hours; but in these it is 

 rare. In many preparations of eggs twenty-two to twenty- 

 four hours old there are present scattered through the yolk, 

 nuclei almost devoid of cytoplasm corresponding in size and 

 general aspect to the secondary yolk cells. (Some of these may 

 be seen in figure 16). While the identity of these cannot be 

 demonstrated it is reasonable to suppose that they are actually the 

 secondary yolk cells, since in their uniform size and poverty in 

 cytoplasm they differ from the majority of the primary yolk 

 cells. These cells, — presumably the secondary yolk cells, — are not 

 infrequently aggregated into clusters, a few of these containing 

 as many as ten or a dozen nuclei. A little later, in eggs twenty- 

 eight to thirty hours old, these almost naked nuclei have disappear- 

 ed. In their place are yolk cells of moderate size, resembling in 

 every respect the smaller representatives of the primary yolk 

 cells. It seems probable therefore that these are the secondary 

 yolk cells which have acquired additional cytoplasm, since there 

 is no other explanation possible of the facts observed. The means 

 by which this increased amount of cytoplasm is acquired is not 

 clear. It may have been manufactured by the metabolic activities 

 of the cell, building cytoplasm out of the food substances of the 

 yolk, or it may have been drawn directly from the protoplasmic 



primary yolk cells by division, and which afterwards contributed to the 

 formation of the fat body and the blood. Heymons (1895) states that 

 these are nothing but blood cells, and are therefore not "yolk cells" 

 at all. 



