228 CHESTER H. HEUSER 



represented on the cast by a prominent mound, but the eleva- 

 tion does not extend as far dorsally or ventrally as do those 

 immediately preceding it. Caudally it blends with the sulcus 

 limitans. Bradley ('04 and '05) described seven neuromeres in 

 a nineteen-day embryo. ^ 



The pontine flexure soon becomes definitely established as 

 shown in figure 5, which is a cast from a 12-mm. embryo. In 

 this embryo the median ventral edge of the cast is very sharp 

 around the flexure and backward toward the spinal cord. At 

 the pontine flexure there are six neuromeral grooves. The rela- 

 tion of the grooves to the median sulcus is clearly shown in the 

 dissection, figure 7, The former seventh groove has been taken 

 up by the sulcus limitans so that it is no longer recognizable. 

 Above the grooves the A^entricle has become quite wide. In 

 dorsal view the body of the ventricle slopes out laterally quite 

 rapidly behind the isthmus, attaining its maximum width in the 

 region of the future lateral recesses, which become distinct in 

 embrj^os of about 15 mm. 



In the 17-mm. embryo the isthmus leading from the mid- 

 brain to the body of the fourth ventricle is diamond-shaped in 

 cross section; the ventral zones are here more extensive than the 

 dorsal, which is the reverse of the condition in the mid-brain. 

 The ventral median sulcus is narrow and deep, so that in the 

 cast there is a sharp edge on the ventral surface of the fourth 

 i^entricle extending around the pontine flexure, and onward cau- 

 dally to the spinal cord. There are now five neuromeres in the 

 hind-brain. The body of the fourth ventricle has expanded a 

 great deal since the last stage described, and the lateral recesses 

 are now well indicated. Slight concavities on either side of the 

 body of the ventricle between the isthmus and the lateral re- 

 cesses, in the dorsal zones, are caused by the thickening of the 

 brain in this region to form the lateral portions of the cerebellum. 

 Behind the lateral recesses the thin roof of the hind-brain has 

 become deeply invaginated to form the chorioid plexus. About 

 100 villi are developed on the fold. 



1 According to Keibel's Normentafel, pig embryos of nineteen days measure 

 from 4.5 to 8 mm. 



