90 The Life-History of Insects 



by the division of the fertilized nucleus into a 

 number of nuclei, which can be recognised scattered 

 through the yolk before any cell-divisions are apparent. 

 Each nucleus is surrounded by a star-shaped mass of 

 protoplasm •, these masses being connected with each 

 other by a network of very fine fibres. The nuclei 

 move towards the margin of the egg (fig. 57, C, D, fi), 

 where they arrange themselves in a layer surrounding 

 the yolk ; their masses of protoplasm become marked 

 off into distinct cells, and a cell-layer, the germ-skin 

 or blastoderm (fig. 57, E), is thus formed internal to 



Fig. 58. — Diagram showing formation of germinal layers. E. ectoderm ; 31. 

 inner layer. Magnified. From Nussbaum, in Miall & Denny's "Cock- 

 roach." 



the yolk-membrane. The stage of development now 

 reached, a hollow bag with cellular envelope is known 

 as the hlastula. 



Germinal Layers. — The formation of the em- 

 bryo begins in an elongate, thickened region of the 

 blastoderm, the primitive streak, which extends along 

 the ventral face of the egg. A groove forms along 

 the centre of the primitive streak resulting in a 

 lengthened inpushing (invagbiation) of the blastoderm, 

 which thus becomes two-layered ; the surface cells 

 overgrow the open edges of the groove, forming an 



