THE DEVELOPMENT OF INSECTS. 



55 



future embryo is tlie formation of the germinal layer, or blas- 

 toderm (from the Greek, meaning primitive skin). This layer 

 is formed at the surface out of a surface-laj^er of larger, often 

 nueleolated, cells which nearly encompass the yolk-mass. At 

 one point there is a break in this cellular layer, and the j'olk 

 granules reach to the surface, so that it appears darker than 

 the other parts of the egg. This cellular layer is soon resolved 

 into the blastoderm, or germinal layer, which thickens and 

 narrows, forming a longitudinal band. This is the first stage 

 of the embryo, which lies as a thin layer of cells upon the outer 

 surface of the yolk. Both ends of the body are alike, and we 

 shall afterwards see that its back lies next to the centre of the 

 egg, its future ventral side looking outwards. The embryo is 

 thus bent on itself backwards. 



In the next stage the blastodenn divides into a certain num- 

 ber of segments, or joints, which appear as indentations in the 

 body of the embryo. The head can now be distinguished from 

 the posterior end chiefly by its larger size, and both it and the 

 tail are folded back upon the body of the embryo, the head 

 especially being sunk backwards down into the yolk-mass. 



In a succeeding stage, as we have observed in the embryo of 

 Diplax, a Dragon-fly (Fig. 57), the head is partially sketched 



Fig. 58. 



out, with the rudiments of the limbs and mouth-parts ; and the 

 sternites, or ventral walls, of the thorax and of the two basal 

 rings of the head appear. The anterior part of the head, in- 

 cluding the so-called "procephalic lobes" overhangs and con- 

 no. 57. Side view of embryo. Tlie procephalic lobes are not shown. 1, antennae; 

 2, mandibles ; 3, maxillae; 4, second ma.\illa! (labium) ; 5-7, lejrs. These numbers 

 and letters are the same in all the figures from 57-(;o. The under-side (sternum) 

 of six segments are indicated. Fig. 58. Ventral view of the same. 



