EMBRYOLOGY OF COCCIDS 97 



6. THE FOR]VL\TION OF THE GERM LAYERS AND THE JEMBRYOXIC 

 ENVELOPES 



At the time the invagination occurs at the posterior end, the 

 blastoderm is one cell thick, except at the posterior end where 

 cell proliferation is evident. The yolk cells (cleavage cells that 

 failed to take part in the formation of the blastoderm) are here 

 and there closely attached to the blastoderm. Now and through- 

 out the embryonic period no evidence of cell migration from the 

 embryo proper into the yolk is noticed. 



The first change occurring after the completion of the blasto- 

 derm is its thinning out in the immediate neighborhood of the 

 posterior pole and also toward the anterior end of the egg, and 

 the simultaneous thickening around the rim of the thinned area 

 at the posterior end of the egg. The thickening near the 

 posterior end is due to the proliferation of cells, as shown in 

 figure 81. The proliferating cells at first are all similar, as in 

 figure 83. The larger and clearer cells found in chains, from 

 the region of the invagination toward the opposite end of the 

 egg, are the germ cells, the history of which will be considered 

 separately. At this stage, then, there are only two kinds of 

 cells within the blastoderm: the germ cells and the cells that 

 form the invaginating germ band. 



In figure 85, which represents a longitudinal section of a some- 

 what older embryo through the region of the blastopore, cells of 

 another or third kind occur between the germ cells and the 

 cells which form the wall of the blastopore. These are the 

 so-called entodermal cells. The cells forming the wall of the 

 invagination and situated next to the entodermal cell mass are 

 cubical or almost columnar and are in direct continuity with 

 the blastoderm layer beyond the' invagination or the blastopore. 

 These are the rudiments of the future ventral plate and amnion. 



A longitudinal section through the blastopore of a similar 

 embryo (fig. 86) shows another, a fourth sort of cells besides the 

 three already mentioned. Although these latter cells may have 

 been present in sections of the egg illustrated in the preceding 

 figure, they have perhaps escaped detection on account of the 

 syncytial nature of the cells at that period. It is at about the 



