XI 



PHYLUM ARTHROPODA 



633 



of a sphere almost parallel with the latter ; eventually they 

 reach the peripheral protoplasm 



at the surface, which then be- 

 comes divided into cell-areas 

 corresponding with the nuclei. 

 The layer of cells thus formed 

 constitutes the blastoderm. This 



vent.pl 



blast 



ser 



amn 



ond ect 



amn.f 



blast 



blast 



ect amn ser 



"blast 



yk. v 



Fiu. 537. A 1), successive stages in the 

 segmentation of the ovum of an Insect. 

 blast, blastoderm ; peri, peripheral proto- 

 plasm ; sey. segmentation-cells ; yk. yolk ; 

 ykc. yolk-cells. (From Korschelt and 

 Heider, after Blochmann.) 



blast 



FIG. 538. A C, transverse sections through 

 the developing ovum of an Insect at suc- 

 cessive stages to show the mode of develop- 

 ment of the germinal layers and of the 

 amnion. amn. amnion ; amn. f. fold of the 

 amnion ; amn. cav. cavity of the amnion ; 

 blast, blastoderm covering the yolk ; ect. 

 ectoderm ; end. endoderm ; gast. invagination 

 of ventral plate ; ser. serosa ; vent. pi. ventral 

 plate ; yk. yolk. (After Korschelt and 

 Heider.) 



thickens along one side to form the ventral plate, as already described 

 in the case of the Cockroach (p. 613), and the changes which this 



