INSECT 'S STOMACH — SNODGEASS 



377 



ample amount of food material for the future embryo (fig. 9 A). 

 The food material is called yolk {Y). The first historic develop- 

 mental stage that the ^g^^ must form is the blastula, but also the 

 yolk must be available at once to the blastula cells, and later to the 

 stomach cells. Hence, the first cells formed by the division of the 

 egg nucleus (A, Nu) disperse to the surface of the yolk and there 

 form a layer surrounding the latter (B). This cell layer is the 

 blastoderm {Bid) and constitutes the blastula of insect development. 

 The future insect, however, is not a product of the entire blastoderm, 

 but of an elongate patch of thickened cells soon differentiated on 

 the under surface of the blastoderm, known as the germ hand (C, 

 D, GB)^ which is the beginning of the true embryo. 



Bid 



D GB 



FiGDRB 9. — Diagrams of early stages in insect development. 



A, an egg of the usual type containing a large quantity of food material, or yolk 

 (Y). B, the enclosure of the yolk in a layer of cells, the blastoderm (Bid), formed 

 by division of the egg nucleus and migration of the resulting nuclei to the periphery 

 of the egg. C, D, cross-section and longthwise section of the egg at a later stage 

 showing the thickened gorm band {GB) on the under surface of the blastoderm. 



Bid, blastoderm ; Cho, the egg shell, or chorion ; DBl, dorsal blastoderm ; OB, germ 

 band ; Nu, egg nucleus ; Y, yolk. 



One of the first events, now, in the further development of the 

 insect embryo is the formation of a groove along the under surface 

 of the germ band (fig, 10 A), the walls of which either sink in 

 bodily (B), or proliferate a mass of cells internally. In either case 

 the germ band becomes two-layered (C) . This developmental process . 

 looks like the formation of an endoderm from the ectoderm, but 

 future events show that the inner laj^er thus formed is mostly meso- 

 derm (C, Msd). However, it has been observed in some cases that 

 from its median part certain cells (B, Vph) migrate into the yolk 

 and appear to have a digestive action on the latter. These cells, 

 therefore, are called " yolk-eaters ", or more technically are known 

 as vitellophags. Moreover, there are also cells distributed along the 

 inner surface of the mesoderm (A, End)^ or particularly aggregated 



