NEUROPTERA AND COLEOPTERA 



299 



Al 



>-t' 



^ 



>,- 





some of the material there but, instead of continuing with the germ cells, 

 wander back into the yolk where they become scattered. In several eggs 

 a mass of dark-staining granular material in the anterior pole of the egg is 

 evident. It is not so extensive as the 

 oosome, but it may be that yolk cells move 

 inward from the anterior pole carrying this 

 material with them. Lassmann (1935) 

 working with the sheep tick has observed 

 that the yolk cells stream inward from both 

 the anterior pole and the posterior pole near 

 the region of the germ cells. A similar 

 situation may exist in this species, although 

 the evidence is not conclusive. After the 

 blastoderm is complete, many cells may be 

 seen migrating back from the periphery as 

 if they were simply crowded out. These 

 correspond to the paracytes of other 

 workers, but there is no evidence to indicate 

 that they disintegrate in the yolk (Fig. 240). 

 When the inner walls of the blastoderm 

 are complete, the blastoderm consists of a 

 thick layer of closely packed columnar cells. 

 The outer ends of these cells, between the 

 nuclei and the periphery, are dense; the inner ends are uniformly 

 vacuolated. The primary dorsal organ appears at this time as a mass 

 of cells that invaginate into the yolk along one side of the blasto- 

 derm leaving a groove on the outside surface (Fig. 241). The 



do 



^n^ 



Fig. 240. — Brachyrhinus. 

 Longitudinal section of forming 

 blastoderm, {cc) Cleavage cell. 







/^.r 



Fig. 241. 



-Brachyrhinus. Cross section of piimary dorsal organ {do). 



groove is very short, but the cellular invagination extends in a longi- 

 tudinal direction for about one-tenth the length of the egg. The ridge of 

 cells is capped by a dark-staining granular mass of cytoplasm. As in 



