EMBRYOLOGY 



153 



four hours a great change is found to have taken place. The 

 whole superficial contents of the egg are at that time arranged in 

 groups, having the appearance of separate rounded or oval masses, 

 pressed together so as to destroy much of their globular symmetry. 

 The egg contents are also divided into very distinct forms, a 

 granular matter, and a large number of transparent globules, 

 these latter being the fatty portion of the yolk ; these are present 

 everywhere, though in the centre there is a space where they are 

 very scanty, and they also do not extend quite to the circuni- 

 ference. But the most remarkable change that has taken place 

 is the appearance in the middle of the field of an area different 

 from the rest in several particulars ; it 

 occupies about one-third of the width 

 and one-third of the length ; it has a 

 whiter and more opaque appearance, 

 and the fat globules in it are fewer in 

 number and more indistinct. This 

 area is afterwards seen to be occupied 

 by the developing embryo, the outlines 

 of which become gradually more dis- 

 tinct. Fig. 83 gives an idea of the 

 appearance of the egg about the middle 

 period of the development. In warm 

 weather the larva emerges from this 

 egg ten or eleven days after it has 

 been deposited. 



The period occupied by the develop- 

 ment of the embryo is very different in 

 the various kinds of Insects; the blowfly 

 embryo is fully developed in less than 

 twenty-fom- hours, while in some of the Orthoptera the embryonic 

 stage may be prolonged through several months. According to 

 Woodworth the blastoderm in Vcmessa antiojM is complete in 

 twenty -four hours after the deposition of the egg, and the 

 involution of the ventral plate is accomplished within three days 

 of deposition. 



Metamorphosis. 



Fig. 83. — A, Egg of Zinmcodes 

 testudo about the middle of 

 tlie development of the em- 

 bryo ; B, micropyles and iiur- 

 rounding sculpture of chorion. 



The ontogeny, or life history of the individual, of Insects is 

 peculiar, inasmuch as a very large part of the development takes 



