The Development of the AUigator 275 



yolk (3^) . The great size of this and the following 

 figure is due partly to the increase in size men- 

 tioned above and partly to the fact that the sections 

 pass through the region of cranial flexure. The 

 present figure (12a) represents the brain cavity as 

 large and dumbbell-shaped, with comparatively 

 thick walls of compactly arranged cells. The 

 ventral end of this cavity (fb) is cut anterior to 

 the region of the optic vesicles, while the dorsal 

 end (mb) may perhaps be called the midbrain. In 

 the sections that follow this one the two cavities 

 are distinct from each other. The medullary 

 canal, as was stated above, is now completely 

 enclosed, except for the ventral opening of the 

 neurenteric canal, to be presently noticed. Sur- 

 rounding the brain is a considerable mass of meso- 

 blast (mes) . It is composed of the typical stellate 

 cells. The ectoderm (ec) is made up of the same 

 irregularly and loosely arranged cells that have 

 been seen in earlier stages; it is of unequal thick- 

 ness in different regions, the thicker parts being at 

 the sides. The amnion (a) has the usual appear- 

 ance, and in this region of coiu^se completely sur- 

 rounds the embryo. 



Figure 12b is ten sections posterior to the 

 section just described. The width of the embryo 

 is greater in this region, but the dorso-ventral 

 diameter is about the same as in the more anterior 

 section. 



The overlying yolk and blastoderm are not 



