132 OUTLINES OF CHORDATE DEVELOPMENT 



Monro. The thickenings of these anterior extensions of the 

 telencephalon and of the regions of the optic thalami and ante- 

 rior commissure produce an apparent straightening of the origi- 

 nal ventral flexure of the brain; this does not really disappear, 

 however, and the infundibulum continues to extend below and 

 in front of the tip of the notochord. 



The hypophysis, while originally not a part of the brain, 

 becomes intimately related with it. This forms very early as a 

 strand of cells extending inward from the inner layer of the sur- 

 face ectoderm (Figs. 37, 40), below the telencephalon and just 

 above the future mouth region (oral plate). These cells multi- 

 ply and form a definite mass lying between the endoderm and 

 the infundibulum, just at the tip of the chorda. This rudiment 

 later cuts off from the ectoderm, and after fusing with the dorsal 

 wall of the pharynx, comes into intimate relation with the 

 lower surface of the infundibulum, forming the essential part 

 of the pituitary body. 



The mesencephalon undergoes only slight modification during 

 the larval period. While its roof and floor remain thin, its 

 ventro-lateral walls thicken as the crura cerebri, connecting with 

 the prosencephalic wall: its dorso-lateral walls form the large 

 founded optic lobes. In front of these the anterior limit of the 

 mid-brain is marked later by the posterior commissure. The 

 cavity of the mesencephalon is called the aqueduct of Sylvius; it 

 becomes narrow and connects the third ventricle with the cavity 

 of the rhombencephalon. 



The rhombencephalon is an elongated part of the brain lying 

 wholly above the notochord. The usual division of this part of 

 the brain into an anterior metencephalon and a posterior myelen- 

 cephalon, is scarcely indicated in the frog. The wide cavity of 

 the rhombencephalon is known as the fourth ventricle; it is con- 

 tinuous anteriorly with the aqueduct of Sylvius and posteriorly 

 with the cavity of the spinal cord. The metencephalon is the 

 region of the cerebellum; this is very small in the frog, and 

 appears late in larval life, as a dorsal and dorso-lateral thicken- 

 ing. Ventrally the walls of the two regions are uniformly 

 continuous. 



