232 EMBRYOLOGY [CH. 



form large cells known as mlellophags, whose function it is to 

 liquefy the yolk and carry it to the developing embryo. They 

 grow to a great size, and possess enormous nuclei. They are not 

 all used up by the time of hatching. Those that are left become 

 finally enclosed, with their store of yolk, in the mid-gut of the 

 young larva, where we have already met with them (p. 71). 



The Ventral Plate (fig. 110, and fig. 109 A). 



The first sign of the Dragonfly embryo is a slight thickening 

 of the blastoderm on the posterior portion of the ventral surface 

 of the egg. This thickening, known as the ventral plate (vp), is 

 caused by proliferation of the blastoderm cells, which here become 

 several layers deep. The ventral plate grows forwards, and quickly 

 becomes a very distinct oval patch, occupying half or more of the 

 ventral surface of the egg. 



Near the posterior end of the plate in that position, in fact, 

 where the thickening first began there 

 next appears a kind of pitting-in or in- 

 vagination of the blastoderm. According 

 to Heymons, this invagination in Libellula 

 (fig. 110) takes the form of a primitive 

 groove (pg) along the mid-longitudinal \Jl0^?^/ K 



line of the ventral plate, running forwards 

 from the point where it first began (known 



as the blastopore). A slight enlargement and pr i m iti v e groove (pg) 

 of the anterior end of this groove marks in Libellula. st stomo 

 ,, ... f .-i f . 11 daeum. After Heymons. 



the position of the future stomodaeal 



opening (st). According to Brandt, in Calopteryx the invagination 

 takes place by a sinking-in of the ventral plate at the blastopore 

 in a gradual manner, beginning with the posterior end (fig. 109 B). 

 The result is the same in either case. 



The cavity formed by the sinking-in of the ventral plate is 

 known as the amniotic cavity. As the ventral plate passes in by 

 the upper lip of the blastopore (B, c), and as undifferentiated 

 blastoderm is drawn in along the lower lip, the walls of the amniotic 

 cavity are dissimilar. The dorsal wall is formed of a single layer of 

 cells, destined to become the amnion (am), while the ventral wall is 

 several layers thick, and is formed by the ventral plate. At the 



