64 P RATHER. [VOL. I. 



polygonal, and with no evident order of arrangement. This 

 fact is strikingly evident, that the limits of the organ, both 

 longitudinally and transversely, mark very accurately the bor- 

 ders of the area over which the brain is in such close con- 

 tact with the hypoblast. 



The mouth cavity posterior to the oral plate is now rather 

 shallow but very wide. Just over the oral epithelium roofing 

 the mouth, on either side of the hypophysis and the infundibu- 

 lum, the internal carotid arteries (v.) are forming, while dorsal 

 to these, in the folds formed between the infundibular lobe and 

 the thalamocoele above, may be seen two other large cavities 

 (?/), the cavities of the preoral somites. 



A sagittal section of a larva about nine days old (Fig. 7) shows 

 the base of the thalamencephalon (fv.) to be a single layer of 

 columnar cells where it rests on the hypophysis, but thickened 

 in front for the chiasma and likewise behind in the infundibu- 

 lar region, which has grown larger. The end of the chorda 

 approaches very near to the infundibulum, but remains separated 

 therefrom by mesoblastic cells, which may be seen to grow in 

 between the brain and mouth roof to the limits of the hypophy- 

 sis at either end. The lower jaw is relatively further back, its 

 tip now lying under the anterior end of the hypophysis. The 

 epithelium of the mouth is continuous beneath the hypophysis, 

 but the basal layer of endoderm appears to continue to divide, 

 adding new cells to the hypophysis by proliferation. The 

 marked difference between the shape and arrangement of the 

 cells in the upper and lower portions of the organ may be 

 noticed still. At the posterior end the body is distinctly 

 separated from the mother layer, while at the anterior end its 

 cells take on more and more the character of the mother layer 

 as we go forwards. The organ measures in this plane 196/11 in 

 length by 41 /JL in thickness. 



A transverse section of 'a slightly older stage (Fig. 8), during 

 the tenth day, shows an oval-shaped organ 130 /-t in breadth by 

 64 /u. in thickness. It is clearly separated from the endoderm 

 at either side, so that it seems to be wholly differentiated, but 

 still lying in a depression in the endoderm caused by the trans- 

 formation of the cells of this layer into cells of the hypophysis. 



