226 CHESTER H. HEUSER 



to the pineal recess. This outgrowth (figs. 14 and 15) which Ues 

 just behind the posterior commissure is the incisura postcommis- 

 surahs. It is not present in the 20-mm. pig but is very distinct 

 in the 45-mm. embryo. The ependymal and mantle layers but 

 not the ectoglial layer of the brain-wall make a distinct outward 

 bend above it. Caudad to the incisure, the dorsal surface of the 

 mid-brain cavity slopes gradually upward to a height of 0.6 mm. 

 and then extends nearly straight backward over the body of the 

 cavity. The inferior collicular recesses are very much more dis- 

 tinct than in younger pigs. 



The length of the mid-brain, including its collicular recesses is 

 5.0 mm. and- the width, in the widest part is 2.6 mm. The vol- 

 ume of the cavity is very much greater than in preceding stages 

 and is now about 6.9 cu. mm. 



A cast of the cavities of the mid-brain in an embryo measur- 

 ing 110 mm. is of an irregular prismatic form as shown in figure 

 24. It presents a median dorsal ridge which extends from the 

 incisura postcommissuralis posteriorly, ending in a median depres- 

 sion between the eminences representing respectively^ the cavi- 

 ties of the right and left inferior colliculi. The outer margin of 

 the cavity of each inferior colliculus is continued forward as a 

 prominent ridge, corresponding with the sulcus lateralis. This 

 ridge flattens out rather sharply and completely a short distance 

 behind the posterior commissure. At the base of the cavity of 

 the inferior colliculus, a prominent ridge ascends from the isth- 

 mus, but it can be followed only a short distance behind the 

 posterior commissure into the territory of the mid-brain, where 

 it terminates. This ridge represents the sulcus limitans of the 

 isthmus. Ventrally there is another ridge which may be regarded 

 as the sulcus limitans of the mid-brain, although it is not con- 

 tinuous with the structure just described as the sulcus limitans 

 of the isthmus. It arises beneath the latter in or near the median 

 ventral sulcus. It then passes laterallj', diverging from its fel- 

 low on the opposite side. The point of widest separation is 

 soon reached, after which the two sulci gradually converge, com- 

 ing together in the mid-ventral line beneath the incisura post- 

 commissuralis. The surfaces between the ridges on the cast are 



