58 



leaving the yolk bodies in the central protoplasmic mass. 

 The nuclei undergo division as they pass to the exterior, 

 and they eventually arrange themselves to form a two- 

 layered blastoderm — the future ectoderm and mesoderm. 

 Of the cells which are left behind in the yolk, some are 

 scattered singly through its substance, and have large 

 and deeplj^-staining nuclei ; the rest have a tendency to 

 remain congregated together in little clusters. The 

 former are the yolk cells or vitellophags, Avliich serve to 

 transform the yolk for the nutrition of the embryo ; the 

 latter, at a subsequent stage in development, form the 

 rudiments of the future endoderm {i/.c. and e.n. in 

 fig. 76). 



About the time of the completion of the blastoderm 

 a modification of its cells takes place at the upper pole of 

 the egg, forming the precephalic or dorsal organ. At 

 this point the cells become markedly columnar, and when 

 viewed in section they form a lens-shaped mass of cells 

 (fig. 7G, 2'X-'- 0.). This remarkable structure is apparently 

 a vestigeal organ ; it soon commences to degenerate, and 

 eventually disappears altogether. AVheeler* homologises 

 this organ with the " dorsal organ '' of certain Crustacea 

 with the iudusium of the Orthopteron X/phitliitm. There 

 are no traces at any stage in the development of structures 

 corresponding with the amnion and serosa of other 

 Insecta. 



The ventral plate or germ band first aj^pears as a 

 narrow area of mesoderm. It is formed by the cells of 

 the latter migrating from their original position and 

 becoming restricted to a band-like zone. This zone, with 

 the exception of where it is interrupted by the dorsal 

 organ, forms a girdle completely encircling the egg. As 

 the result of this migration of the mesoderm, the greater 



* Loc. cit. p. 55, Vid<j also Willey. Quart. Juurn. Mic. Sci., xii, 1899. 



