increase naturally affecting the embryonic ectoderm, which always 

 remains slightly wider than the underlying mesodermal tissues. 

 During the two succeeding stages, XII and XIII (Figs. 81 and 

 82), the embryo continues to increase in breadth, its lateral limits 

 being marked by the lateral now dorsal margins of the em- 

 bryonic ectoderm which is progressing upward over the sides 

 of the egg toward the dorsal mid-line. This growth of the em- 

 bryonic ectoderm, as the figures indicate, takes place in part at 

 the expense of the extra-embryonic ectoderm, which, with the 

 increasing breadth of the embryonic ectoderm, undergoes a cor- 

 responding decrease. Meanwhile the embryonic ectoderm is 

 steadily diminishing in thickness, and at its margin it passes 

 rather gradually into the thinner extra-embryonic part. Between 

 Stages XIII and XIV, the lateral margins of the embryo meet 

 and unite along the dorsal mid-line, the extra-embryonic ectoderm 

 being now entirely absorbed by the embryonic ectoderm, now the 

 definitive hypodermis (Fig. 82). 



The head capsule is formed as follows : During the interval 

 between Stages VII and VIII, when the cephalic end of the germ 

 band, consisting principally of the protocerebral lobes, travels 

 around the cephalic end of the egg, the interval between the 

 lateral margins of the procephalic lobes and of the adjoining 

 margins of the gnathal segments remains continuously filled by 

 ectoderm. The interval between the procephalic lobes is, at 

 Stage VII, filled by a thick layer of ectoderm (Fig. 29, Ect), and 

 after the procephalic lobes reach the dorsal side of the egg, 

 this part of the ectoderm constitutes the dorsal part of the head 

 capsule, comprising the triangular area between the cerebral 

 lobes. The remainder of the head capsule is formed from the 

 superficial layer of that part of the cephalic ectoderm concerned 

 in the formation of the brain. 



