CEREBRAL VENTRICLES IN THE PIG 229 



In a 22-nim. embryo (figs. 11 and 12) the fundamental arrange- 

 ment of parts has not been altered. The cerebellar thickenings 

 have enlarged considerably, producing correspondingly greater 

 depressions in the dorsal zones along the body of the fourth 

 ventricle. The sulcus limitans, for the same reason, has been 

 made verj^ much more evident. The lateral recesses extend fur- 

 ther to each side and are comparatively much narrower than in 

 the younger stage considered. The chorioidal lamina extends out 

 to the ends of the lateral recesses, and the number of villi spring- 

 ing from it has increased to about 150. Caudad to the chorioid 

 plexus the dorsal part of the cavity is slightly distended or puffed 

 up dorsally, thus forming the 'caudal protrusion' (Blake '00). 

 In advanced embryos of dogs, cats, pigs, sheep and also the 

 chick, according to Blake, there is a marked caudal protrusion. 

 He wrote, ''This protrusion is completely closed and resembles 

 the finger of a glove." It can hardly be described in the pig 

 as 'finger-like' since, as shown in figure 11, it forms a rounded 

 dome. 



Taken as a whole, the fourth ventricle in the 45-mm. pig 

 (figs. 14 and 15) has the same parts as seen in the 22-mm. embryo. 

 The cavity of the isthmus has increased in dorso-ventral diame- 

 ter from 0.75 to 0.85 mm. As seen by comparing figures 11 

 and 14, it has become relatively low, but laterally it has become 

 further expanded. Its thin lateral edge, as seen in the cast (fig. 

 15), extends backward and becomes continuous with the body 

 of the ventricle a short distance in front of the lateral recess. 

 Where this thin edge fuses with the body there are three small 

 wrinkles — the lateral remnants of the neuromeral grooves. The 

 two halves of the cerebellum have become very much thicker 

 and meet at an angle of about 130°. The medial part of the 

 cerebellum is also thicker than before, so that all together the 

 broad median fissure of younger stages has been appreciably 

 reduced. The lateral recess, while absolutely larger than before, 

 appears much slimmer. The floor and roof of the recess have 

 almost come together, the cavity being largely filled with the 

 chorioid plexus. In figure 23, in the left recess, is seen a long 

 stretch of the chorioid fold. Sections a short distance caudad 



