236 THE EMBRYOLOGY OF THE HONEY BEE 



direct genetic connection between the dark-stained spherules 

 and the adjoining nuclei, since in every case a small but appre- 

 ciable space separates the two kinds of nuclei. Moreover the 

 nuclear membrane of the normal nucleus is always intact at 

 every point. It may be assumed therefore that all of the sub- 

 spherical deeply staining masses observed within the yolk are 

 formed by ordinary degeneration of yolk cells. At the stages 

 beyond eighteen to twenty hours up to the time of the formation of 

 the germ layers, densely stained chromatic spherules are commonly 

 found isolated within the yolk, sometimes appearing as though 

 caught in the meshes of the protoplasmic network (Fig. 94E), 

 sometimes surrounded by a fragment of cytoplasm (Fig. 94F). 

 That these are derived from yolk cells is evident by comparing 

 them with those still enclosed within the yolk cells (Fig. 94D). 

 In addition, it is possible to find a spherule in the act of leaving 

 a yolk cell (Fig. 946), as well as other stages in the emigration 

 of the spherules out into the yolk (Fig. 94, A and C). The 

 chromatin spherules are therefore equivalent to the "entodermal 

 paracytoids" described by Friederichs (1906) in Donacia, since 

 they have precisely the same history. 



Shortly after mitotic division in the yolk cells ceases, and when 

 the nuclei of the ventral region of the blastoderm have become 

 arranged in a double layer, (18-20 hours) not a few of these 

 nuclei may frequently be detected in the central margin of the 

 blastoderm, at some distance from their fellows. Others may 

 be seen half in the blastoderm, and half in the remains of the 

 peripheral protoplasmic layer lining the blastoderm while still 

 others are entirely within this layer (Fig. 95). Favorable prep- 

 arations show a considerable number of nuclei in all these stages. 

 The migrating nuclei apparently carry with them little or no 

 cytoplasm and after leaving the blastoderm become embedded in 

 the dark-stained vestiges of the peripheral layer of cytoplasm. 

 Henceforward they can no longer be distinguished with certainty 

 from the smaller representatives of the primary yolk cells. The 

 yolk cells thus produced by migration from the blastoderm may 

 be termed the secondary yolk cells 2 * by reason of the later time of 



21 This term has already been applied by Cholodkowsky (1891) to certain 

 cells in the cockroach embryo which he believed were derived from the 



