600 



ZOOLOGY 



SECT. 



"blast 



ser 



amn 



amn.f 



The layer of cells thus formed constitutes the blastoderm. This 



thickens along one side to 

 vent ol f rm the ventral plate, as 



gast ~ 7 ^^-^^^^^' already described in the case 



of the Cockroach (p. 581), and 

 the changes which this struc- 

 ture undergoes, together with 

 the mode of formation of the 

 appendages, are similar in 

 most members of the class. 

 The same holds good of the 

 formation of the amnion and 

 the development of the en- 

 doderm. In some cases there 

 is developed between the 

 serosa and the true amnion 

 a space filled with yolk, and 

 the ventral plate appears sunk 

 within the yolk. The nervous 

 system is developed from the 

 ectoderm in the manner in- 

 dicated in the account of the 

 development of the Cockroach 

 (p. 582). The tracheal system 

 is derived from a series of 

 pairs of segmentally arranged 

 ectodermal involutions (Fig. 

 492, st.) 



Metamorphosis. In some 

 instances the young Insect, 

 when it escapes from the 

 egg-membranes, has exactly 

 the form of the parent, ex- 

 cept that, as a rule, the 

 wings have not yet grown. 

 But in most cases there is a 

 metamorphosis. In some this 

 is comparatively slight and 

 gradual, the adult Insect dif- 

 fering from the larva only in 

 comparatively unimportant 

 points, and the segments and 

 appendages of the latter be- 

 coming directly converted into 

 those of the former. Such a 

 metamorphosis, in which there 



is no quiescent stage, is said to be incomplete. The term complete 



blast 



ect 



amn ser 



end 



"blast 



FIG. 490. A C, transverse sections through the 

 developing ovum of an Insect at successive 

 stages to show the mode of development of 

 the germinal layers and of the amnion. 

 an, 1 1. amnion; uian.f. fold of the amnion; 

 uinn. car. cavity of the amnion ; blast, blasto- 

 derm covering the yolk ; cct. ectoderm ; end. 

 endoderm ; yast, invagination of ventral 

 plate; .<(./. serosa; rent. pi. ventral plate; 

 yk. yolk. (After Korschelt and H eider.) 



