DEVELOPMENT OF THE NERVOUS SYSTEM 537 



The mid-brain 



Like the fore-brain, the mid-brain at the time of hatching, is a 

 massive structm-e, the cavity being reduced to a narrow slit. At 

 this stage, a deep groove extends from the caudal recess of the 

 mid-brain to the mesencephahc pit (fig. 35) . In figure 33 is seen a 

 depression — the first indication of this groove — in the dorso-caudal 

 portion of the mid-brain. It becomes very prominent, but shortly 

 after hatching, as shown by a 13 mm. larva (figs. 46 and 47) the 

 cavity broadens out and this groove disappears. At this time also 

 a distinct sulcus limitans becomes visible. 



In a 13 mm. larva, and to a less extent in a 23 mm. specimen, the 

 cinerea extends to the surface in the medial line. Not until after 

 transformation is a zone of alba present between it and the surface. 

 Figure 57 shows a division of the cinerea into an ectal and ental 

 layer by a zone of alba. 



The cerebellum 



The separation of the cerebellum from the medulla, evident at 

 an early stage, can be distinguished by a dorsal constriction 

 between them and by a thinning of the roof from this groove 

 caudad. 



The cerebellum is a greatly differentiated segment of the brain. 

 In Stage XII (fig. 35) it has assumed nearly its adult shape. It is 

 composed of a commissural band of fibers surrounded by a small 

 amount of gray m.atter. The gray matter in later stages shows a 

 bilobed condition (consult figs. 47, 53 and 58). 



