230 EMBRYOLOGY [CH. 



egg from contact with the air. This forms a more or less regular 

 thin layer over the chorion. It is slightly thickened at the posterior 

 pole. At the anterior pole it is very greatly thickened, forming 

 a conical cap, known as the pedicel, by the apex of which attachment 

 is made to the egg-string. The surface of this cap shews the impress 

 of the follicle cells more plainly than the chorion does. 



The egg has a dorsal and a ventral side, the former being 

 distinctly convex, the latter much flatter. The terms dorsal and 

 ventral, applied to these surfaces, indicate the future position of 

 the late embryo, but not of the early embryo, which is reversed. 



The processes of maturation, fertilization and extrusion of the 

 polar bodies have not been followed in the Dragonfly's egg, the 

 immense mass of yolk making the investigation very difficult. 



Embryonic Development (figs. 109-112). 



The present state of our knowledge of the embryology of the 

 Dragonfly is due to Brandt [13], Heymons [71] and Tschuproff [isi]. 

 The work of Brandt on Calopteryx still remains the most valuable 

 source of our knowledge. The other two authors have chiefly 

 investigated problems concerning the germinal layers; their 

 studies were made on Libellula and Epitheca. Much still 

 remains to be done, particularly with regard to the early seg- 

 mentation, and the formation of the various internal organs in 

 later embryonic life. 



Early Stages. 



Cleavage or Segmentation of the Egg. The first segmentation- 

 nucleus is formed, as in all insects, by union of the male and female 

 pronuclei near the surface of the yolk. This nucleus then moves 

 inwards, and sub-divides into a large number of daughter-nuclei. 

 These nuclei then move outwards to the surface of the yolk, 

 where each takes up a position surrounded by a small "island" 

 of protoplasm. These islands are all actually connected together 

 by fine threads. They finally fuse with the periplasm, and become 

 marked off by constrictions between them, so that they soon take 

 on the appearance of a single row of surface cells. This is the 

 blastoderm. Since the cells are formed at the surface, this form 

 of segmentation is known as superficial. 



