xi PHYLUM ARTHROPODA 581 



inferred from what is known of other Insects that the nucleus with 

 a small quantity of protoplasm lies enclosed within the food-yolk. 

 It undergoes division (Fig. 464), and some of the resulting cells 

 travel to the surface, where they form an investing layer the 

 blastoderm (blast.), while others the yolk cells (yk. c.\ remain 

 scattered in the interior of the yolk. 



On the ventral side there is soon formed a thickening of the 

 blastoderm, owing to the cells in this situation becoming columnar ; 

 this forms what is termed the ventral plate (Fig. 465). In front 

 this is wider (in the position of the future head) than it is behind. 

 It becomes divided by a number of narrow transverse lines which 

 indicate the boundaries of the future segments. 



Rudiments of appendages appear on the head and thorax, 

 and a series also appear on the abdomen, which, however, 

 subsequently disappear. The segment on 

 which the rudiments of the antennae appear /^^ 



is at first post-oral in position, but subse- / \ 



quently becomes fused with a prse-oral seg- 

 ment (prostomium), so that the antennas i ^ ~ 

 acquire their permanent prse-oral position 

 only secondarily. The prostomial segment, 

 the antennary segment, a segment devoid of 

 appendages, the segment bearing the rudi- 

 mentary mandibles, and those bearing the fe-^ j 

 two pairs of maxillae, all unite to form the 

 head of the adult. The ventral plate, which ' I 

 was superficially situated when first de- 

 veloped, becomes gradually sunk within the FlG< 4 65.-ventrai plate of 

 substance of the yolk, and thus becomes e "l^ ryo , c c k roach 



/ ' . with developing ap- 



separated from the chonon by a layer of pendages. (After Mian 



yolk. On this follows the appearance of the 



larval membranes. On either side arises a 



fold of the blastoderm (Fig. 466, amn.f.); and the two folds 



gr.ow inwards, and eventually unite over the body of the embryo, 



forming a complete two-layered covering for it. The outer layer 



is termed serosa (ser.), the inner amnion 1 (amn.) 



Along the middle of the ventral plate there soon appears a 

 groove the germinal groove. This grows downwards, and forms 

 a tube, which becomes completely detached from the ectoderm. 

 The lumen of the tube becomes filled up with cells, and the solid 

 strand thus formed divides longitudinally into two parts the 

 mesoderm bands. There is some doubt as to whether the endoderm 

 is also formed in the course of this invagination, or by modification 

 of the yolk-cells. Infoldings of the ectoderm at the anterior and 



1 This term is derived from the Vertebrata, in which there is an analogous 

 membrane, occupying, however, a dorsal instead of a ventral position as regards 

 the body of the embryo. 







