234 



CHESTER H. HEUSER 



by the growth of the walls — being interrupted at the junction 

 of the isthmus with the mid-brain. 



In the casts of the ventricles of the pig, in early stages, there 

 is a broad line of maximum width corresponding with the sulcus 

 limitans. 



s.i. 



Fig. 2 Sections through the hind-brain of pig embryos, {A, 3.9 mm. ; B, 10 mm. ; 

 C, 23.6 mm.), showing the change in position of the sulcus limitans. 



This groove which is lateral in the cord becomes a ventral 

 groove in the medulla as described by His, and at the junction 

 of the isthmus with the mid-brain it comes to an end. On account 

 of the expansion of the roof-plate in the hind-brain the sulcus 

 limitans soon falls below the line of maximum width. Thus in 

 the diagram (fig. 2, B) the sulci are ventrally placed, and the 

 width of the fourth ventricle between them is less than it is 

 slightly further dorsally. In an older embryo (fig. 2, C) the 

 sulci are close together and on the floor of the ventricle. In 

 the mid-brain there is early a line of greatest width, which becomes 

 shifted so as to form the outer border of the tegmental fold, as 

 shown in the diagram, figure 3. 



■ S. I. 



Fig. 3 Sections through the mid-brain of pig embryos, (A, 3.9 mm.; B, 23.6 

 mm.; C, 110 mm.); showing the change in position of the sulcus limitans. 



