3o8 Natural History Bulletin. 



down toward the phaiynx. The neck of the flask has sepa- 

 rated from the epithelium lining the pharynx, owing to the 

 separation of this Hning from the mesoblast. Within the 

 lumen of the flask is a mass of structureless matter, without 

 evidence of nuclei. The hasmatoxylin has stained this to 

 some extent, one patch taking rather a deep stain. The neck 

 and the lower portion of the lumen in this flask contain 

 many cells, so that the open communication which previously 

 existed between the cavity of the flask and the pharynx is 

 now obliterated. 



Surrounding the hypophysis cerebri is a structureless mem- 

 brane, which is quite conspicuous between the flask and the 

 infundibulum, and also between the flask and the processus 

 infundibuH. The mesoblastic cells are crowded back by the 

 growth of the hypophysis cerebri. 



The peculiar nest of mesoblastic cells so clearly seen in 

 figure 4, has entirely disappeared in this embryo, and with it 

 all traces of Seessel's pocket. 



Resting on the flattened side of the h3^poph3^sis cerebri, 

 and separated from it by a thin membrane, is the layer of 

 cells which forms the boundary of the infundibulum. The 

 membrane which separates these two epiblastic layers is 

 probably derived from the mesoblast, but beyond this the 

 two epiblastic layers are in contact. In figure 4 these two 

 layers are in actual contact, no membrane intervening. 



Above the hypophysis cerebri, and resting on its base, is 

 an elongated body, the processus infundibuH (P. I). This is 

 also separated from the hypophysis cerebri by the membrane 

 previously mentioned. The processus infundibuH has ad- 

 vanced into the mesoblast until it approaches the nervous 

 lining of the posterior cerebral vesicle. From all that can be 

 seen in figure 6, the processus infundibuH is separated from 

 the infundibulum, but this appearance is due to the fact that 

 the section did not lie in the median plane. 



Figures 7 and 8 are taken from the same embryo as that 

 from which figures 5 and 6 were taken. Figure 7 shows the 

 communication between the ccelise of the anterior cerebral 



