XI 



PHYLUM ARTHROPODA 



629 



peri 



blast 



distributed through a considerable portion of the yolk. These 

 (Fig. 502) all migrate to the surface, where they multiply rapidly 

 and form a layer, the blastoderm, which becomes thickened along 

 the ventral surface by the cells being elongated in a vertical 

 direction. From the blastoderm a number of cells pass inwards 

 into the substance of the 

 yolk, where their function is 

 to convert the yolk- material 

 into various soluble substances 

 for the nourishment of the 

 blastoderm. The ventral thick- 

 ening of the latter is the ven- 

 tral plate : its cells prolifer- 

 ate, and the plate comes to 

 be several cells thick: in front 

 it becomes broader an indi- 

 cation of the position of the 

 future head-lobes. At the op- 

 posite end there is a specially 

 thickened area of the ventral 

 plate with a slight depression 

 on its surface ; the depression 

 perhaps represents the blas- 

 topore, since it is from this 

 point forwards that the for- 

 mation of the mesoderm pro- 

 ceeds. The latter is formed 

 as a longitudinal band which 

 bifurcates in front in the 

 position of the head-lobes. 



The mode of origin of the 

 endoderm in the Cockroach 

 is not known with certainty. 

 It appears beneath the meso- 

 derm, in two separate portions, 

 as a thin layer of cells one 

 portion, the anterior, coming 

 into relation with the begin- 

 nings of the stomodseum, 

 which arises as an in vagi na- 

 tion from the surface in the 



region of the head-lobes and the other, the posterior, uniting 

 with the proctodseum, a similar ectodermal invagination at 

 the posterior end of the ventral plate. These two rudiments 

 of the endoderm grow towards one another, and eventually 

 meet to form a continuous layer destined to form the wall of 

 the mesenteron. The ventral plate early becomes divided by a 

 VOL. i s s 



Wast 



yk.c 



FIG. 502. A D, successive stages in the seg- 

 mentation of the ovum of an Insect ; blast. 

 blastoderm ; peri, peripheral protoplasm ; seg. 

 segmentation cells ; iik. yolk ; yt. c. yolk-cells. 

 (From Korscheltand Heider, after Blochrnann.) 



