THE BRAIN 



149 



vein (Lillie, igo8, p. 368) was dissected from the mid-ventral line and is drawn (Plate VI, 

 fig. I, uv) pulled over to the left side with a piece of the body- wall to which it was 

 attached. The latter is cup-shaped because it enveloped a protuberance on the ventral 

 side of the yolk sac. 



COMPARATIVE STUDIES IN DEVELOPMENT 



THE BRAIN 

 This commences in penguin embryos, as in most other vertebrates, with the formation 

 of longitudinal medullary folds. They are wider apart both anteriorly and posteriorly 

 than the corresponding stage in the development of the fowl. The medullary folds 

 quickly unite to form a neural tube out of which the compartments of the brain dif- 

 ferentiate. By the time stage 31 is reached, i.e. NT 31, the hemispheres, optic lobes, and 



w 



XL--- 



SI Y 'm 



Fig. 4. Right side of the brain of Gentoo penguin embryos : a, stage 3 1 ; Z>, stage 34; c, stage 37, drawn from 

 the brain of a Ring penguin embryo. 



ce, cerebellum; hm, hemisphere; o, optic lobe; me, medulla oblongata; pi, pineal body; I-XU, roots ol 



cranial nerves. 



