3o6 Natural History Bulletin. 



notochord. The notochord in this embryo is not a straigfht 

 rod, but is somewhat undulating, and for this reason but a 

 small portion appears in this section. The cells of the noto- 

 chord are usually spherical, but many are irregular, owing to 

 mutual pressure. Many of them contain two nuclei. 



Lying on the ventral side of the notochord, and forming 

 the prominence between the Seessel's pocket and the hypo- 

 ph3-sis cerebri, is the peculiar nest of mesoblastic cells to 

 "which reference has been made. The cells forming this nest 

 are nucleated, and are not crowded, although they are not so 

 far apart as the branched mesoblastic cells. This nest of 

 cells very strongly resembles an epithelioma nest. From the 

 appearance of the branched mesoblastic cells immediately sur- 

 roundin<T this mass, it seems that the cells forming the nest 

 have been proliferating quite rapidly. The formation of 

 Seessel's pocket and apparently the initiatory step in the 

 formation of the hypophysis cerebri are due to the growth of 

 this nest of cells. The next section to the one from which 

 figure 4 \vas taken shows very distinctly a connection between 

 the notochord and this nest of cells. 



The epithelium lining the posterior pharynx consists of a 

 single layer of cells. As the epithelium passes up to line 

 Seessel's pocket it becomes thickened, and as it is reflected 

 over the nest it ceases to be simple and becomes several cells 

 in thickness. In the apex of the invagination for the hypo- 

 physis cerebri the epithelium is thickest. 



Nothing has been found in any of my sections indicating 

 the remains of the oral plate, or the point of juncture of 

 the hypoblast and epiblast; but from previous observations 

 (Minot, Human Embr3'ology, figures 107 and 170) there 

 seems little room for doubting that the nest (N) indicates the 

 position of the dorsal attachment of the oral plate. 



In figure 2 the infundibulum appears as a long finger-like 

 process of the middle cerebral vesicle. Figure 4 sliow^s the 

 infundibulum as a broad sinus (I). At the upper end of the 

 main sinus is another sinus, which, from the arrangement of 



