CEREBRAL VENTRICLES IN THE PIG 225 



neither the mid-dorsal nor the mid-ventral line is indented at 

 this point. 



Figure 9, of a 17-mm. embryo, shows that the dorsal surface 

 of the mid-brain has become flatter than before and its caudal 

 end projects further toward the hind-brain. This projection, on 

 either side, marks the position of the future inferior colliculus. 

 The ventricle of the mid-brain is somewhat quadrilateral in cross 

 section, and in the cast it shows a prominent longitudinal ridge 

 corresponding with the line of maximum width. The groove in 

 the wall of the brain corresponding with the ridge may be referred 

 to as the lateral sulcus (sulcus lateralis internus). It must not 

 be confounded with the sulcus limitans which, in the cast, is a 

 much less conspicuous ridge, more ventrally placed. The sulcus 

 limitans is more evident in the dissection, figure 8. It separates 

 a small thick-walled ventral zone from an extensive thin-walled 

 dorsal zone. The ventral zones, on either side of the mid-ventral 

 sulcus, form low rounded eminences projecting into the cavity 

 of the ventricle which constitutes the tegmental fold of His. The 

 lateral sulcus passes longitudinally through the dorsal zones, 

 terminating on either side, in the projection which forms the in- 

 ferior colliculus. 



In a slightly older embryo (22 mm.) the dorsal wall of the 

 mid-brain has become relatively thinner, and its ventricle has 

 expanded greatly. At this stage it forms a very conspicuous por- 

 tion of the whole ventricular system. Its volume is estimated 

 at 4.1 cu. mm. Separate recesses for the inferior colliculi can 

 now be recognized. In earlier stages the posterior overhanging 

 portion of the mid-brain ended abruptly in a straight transverse 

 line. In the 45-mm. embryo this border shows a deep median 

 cleft, and each arm of the bifurcation thus formed represents the 

 cavity of an inferior colliculus. In the 22-mm. specimen the cavi- 

 ties projecting backward from the dorso-lateral corners of the mid- 

 brain ventricle have just begun to develop. 



In the 45-mm. embryo the third ventricle is connected with 

 the cavity of the mid-brain by a more elongated constricted por- 

 tion than in previous stages. At the anterior end of the mid- 

 brain ventricle there is a median dorsal extension 1.2 mm. caudad 



