The Hypophysis Cerebri. 305 



axis of mesoblastic cells. Blood channels, containing nucle- 

 ated blood cells, are seen lying in this mass of branched cells. 



Upon the dorsal surface of this mesoblastic axis are live 

 prolongations, reaching almost to the coelia of the posterior 

 cerebral vesicle. Between these prolongations the epiblast is 

 arranged in pockets. Upon the ventral side of the mesoblastic 

 axis is a projection, containing a nest of mesoblastic cells (X) 

 which will be described later. This projection (X) marks 

 the position of the oral plate, or septum which, previous to 

 this, separated the stomodfeum from the mesenteron. 



On the pharyngeal side of the nest just described is a sinus 

 known as Seessel's pocket (S. P). On the oral side of the 

 nest is another pocket, which is the earliest indication of the 

 hypophysis cerebri ( H. C). Projecting from the coelia of the 

 middle cerebral vesicle towards the hypophysis cerebri and 

 almost touching its apex is a conical space, the infundibulum, 

 (I). In tigure 2 it appears to be formed bv the proliferation 

 of ceils on its ventral side, but a study of other sections in this 

 series demonstrates that these epiblastic cells are a portion of 

 the constriction between the anterior and middle cerebral 

 vesicles. 



From the infundibulum another broad shallow sinus is given 

 off, which pushes into the mesoblastic axis. This is the earliest 

 indication of the processus infundibuli (P.I). 



Figure 3 is from another section of the same embrvo. It 

 shows the region of the constriction between the posterior 

 'and middle cerebral vesicles, and also indicates the line of 

 boundary between the middle and anterior cerebral vesicles. 

 The section was not from the median longitudinal plane, and 

 for this reason the structures under consideration in this paper 

 are not clearly shown. 



Figure 4 is an enlarged view of the region under consider- 

 ation, taken from another section in the same series. Just 

 above Seessel's pocket, in the mesoblastic axis, is a mass of 

 rounded cells closely aggregated, and enclosed bv a thin 

 membrane (Xot). This mass is the cephalic extremity of the 



