118 



Fig. 4. 



mW: 







;ii^ 



•fi^ 





i£ 



Fig. 6. 



Fig. 7. 





Fip. 4. Embryo at about the 25th hour of incubation. Reference letters as iu 

 previous figs. 



Fig. 5. Embryo in about the 25th hour of incubation showing the beginning of 

 the decline of the "accessory optic vesicles". 



Fig. 6. Embryo at about the 26th hour of incubation showing the reduction of 

 the accessory vesicles to four pairs. 



Fig. 7. Embryo at about the 27th hour of incubation showing the reduction of 

 the "accessory optic vesicles" to two pairs which are destined to rapidly disappear. 



not only widened, but, the bulgings show: a tendency to extend back- 

 wards, and as the optic vesicles take shape, this elongated condition 

 is replaced by a shorter and more rounded bulging. The diverticulum, 

 which is the beginning of the optic vesicle, is evidently due to unequal 

 growth of the brain walls. — There is a more rapid cellular division 

 in the epithelium, which is taking the course of becoming sensory, 

 and this region of ditferention exhibits a tendency to extend backwards, 

 and involve not alone the anterior end of the neural epithelium, but 



