MIDBRAIN >i;} 



Baker and Graves (S*^) described the external form of the brain in 

 six stages of A. jeffersonianum (3-17 'mm.). Their figures 1 and 2 show 

 that in their 3-mm. embryo, which is 7 days and 16 hours old and 

 in a premotile stage, the mesencephalic flexure has begun. In Harri- 

 son's stage 30 of A. punctatum, about 24 hours younger than Cog- 

 hill's nonmotile stage, this flexure is far advanced ('38, fig. 1), and 

 the reverse flexure leading toward the evagination of the cerebral 

 hemisphere is incipient. 



Correlated with these changes in external form, cells are proliferat- 

 ing internally, and the fibrous connections are established. CoghilFs 

 charts show that differentiation is precocious in the peduncle, the 

 chiasma ridge, and the olfactory area of the hemisphere. Other local 

 areas of differentiation appear successively, and, before the early 

 swimming stage is reached, confluence of these areas has begun, and 

 fibers are descending from the peduncle and ventral thalamus in the 

 primary motor tract, which is the precursor of the fasciculus longi- 

 tudinalis medialis. This development is well advanced before connec- 

 tion is made between the sensory field of the tectum and the motor 

 field of the peduncle (compare '37, figs. 1 and 2; '38, figs. 5 and 20). 

 Fibers from the retina enter the brain in early swimming stages, but 

 they do not reach the tectum until the close of this period. These op- 

 tic fibers enter the tectum at its anterior end in the eminence of the 

 posterior commissure, and the first connection between optic tectum 

 and peduncle is by way of this commissure, in which fibers first ap- 

 pear in the coil stage (Harrison's stage 35), long before optic fibers 

 reach the tectum. 



The commissura tecti in the remainder of the midbrain roof ap- 

 pears much later, beginning as an extension spinalward from the 

 posterior commissure. As late as the midlarval period (A. tigrinum 

 of 38 mm.) this commissure has matured only in the anterior half 

 of the tectum ('39a, figs. 1, 11, 12; '39/;, fig. 1). Full details of the 

 development of the optic nerve and its connections have been pub- 

 lished ('41), as well as some features of the development of the 

 tectum ('42, p. 243). 



In development the nucleus posterior tecti is retarded as compared 

 with the tectum opticum, more so in A. tigrinum than in A. puncta- 

 tum ('386, p. 413; '39a, p. 265). The ventricle is here dilated as the 

 recessus posterior mesencephali, and in larval stages the deep sulcus 

 isthmi ends dorsally in this recess ('38, fig. 18; '14a, fig. 2). The roof 

 and side walls of the posterior recess remain relatively thin until late 



