EXPERIMENTS WITH CHRYSOMELID BEETLES. 



239 



nuclei of one group form a more or less regular layer equidis- 

 tant from the periphery, and later migrate outward, fuse with 

 the " Keimhautblastem " and become the blastoderm (Fig. 2, bT). 

 The nuclei of the other group remain behind in the yolk (Fig. 2, 

 v), which it is their duty to dissolve. Eight of the cleavage 

 products which reach the posterior end do not help to form the 

 blastoderm, but gather the germ cell determinants about them 

 and continue their migration until they are entirely separated 

 from the egg; these are the primordial germ cells (Fig. 2, pgc). 



Pgc 



FIG. 2. A longitudinal section through an egg of Leptinotarsa decemlineata 

 one day after deposition when in the blastoderm stage, bl, blastoderm; pgc, 

 primordial germ cells; v, vitellophag; y, yolk. 



FIG. 3. Superficial view of the right side of an egg of Leptinotarsa decemlineata 

 thirty-six hours after deposition, gb, germ band; pgc, primordial germ cells; pi, 

 procephalic lobes; s, stomodeum; vg, ventral groove. 



FIG. 4. Surface view of right side of an egg of Leptinotarsa decemlineata forty- 

 eight hours after deposition, a, antenna; in, mandible; ?', first maxilla; m 2 , second 

 maxilla; t'-l 3 , thoracic appendages; //, tail fold. 



They remain quiescent for a considerable period and then 

 migrate into the embryo, which has developed in the meantime. 

 At the end of two days the germ band appears on the ventral 

 surface of the egg (Fig. 3, gb); this lengthens within the next 

 twenty-four hours, growing forward to the extreme anterior end, 

 and posteriorly until the tail-fold reaches over half way up on 

 the dorsal surface (Fig. 4, tf). During this elongation the embryo 

 segments and the appendages of the head and thorax grow out. 

 The entire embryo then contracts antero-posteriorly and begins 

 to grow around the yolk (Fig. 5). This contraction continues 



