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 antennas appear 

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

 quently becomes fused with a prse-oral seg- 

 ment (prostomium), so that the antennae 

 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 

 two pairs of maxillae, all unite to form the 

 head of the adult. The ventral plate, which 

 was superficially situated when first de- 

 veloped, becomes gradually sunk within the 

 substance of the yolk, and thus becomes 

 separated from the chorion by a layer of 

 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 

 grow inwards, and eventually unite over the body of the embryo, 

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

 is termed serosa (scr.), 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 mvagination, 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. 



Fie. 4'i.j. Ventral plate of 

 embryo Cockroach 

 with developing ap- 

 pendages. (After Miall 

 and Denny.) 



