62 P RATHER. [VOL. I. 



rior part of the thalamencephalon as far as the posterior limit 

 of the optic chiasma (oc.}. Here, immediately under the central 

 part of the thalamencephalon (lpo.\ the hypoblastic cells may 

 be seen to have assumed a different shape and size, and the 

 number of layers to have increased. From roundish or ovoid 

 cells they become long, spindle-shaped, much smaller, and 

 arranged in crescent-shaped layers about six in number and fit- 

 ting one within another, so that the whole mass of cells over 

 a region about 180 yu, in length is nested in this peculiar manner. 

 This is the first trace of the hypophysis (hy.) recognizable. 

 Back of this differentiation the cells of the endoderm are again 

 of the same rounded or ovoid shape as in front and arranged in 

 two layers. Thus, I repeat, there can be detected no fold or 

 overgrowth of ectoderm to give rise to the hypophysis nor 

 evagination of the endoderm. Its cells are differentiated in 

 situ apparently by longitudinal division of the cells consti- 

 tuting the roof of the foregut. 



The chorda at this stage extends forwards under the hind 

 brain to near the tip of the infundibular fold, which has greatly 

 enlarged. The base of the thalamencephalon has elongated and 

 is now a single layer of cells (Ipo.} posterior to the chiasma, rest- 

 ing closely upon the dorsal wall of the foregut. 



In a larva early in the eighth day, shortly after hatching, a 

 sagittal section (Fig. 4) shows the oral plate broken in the 

 center. But sections of the series on either side show the 

 membrane or its remnants still intact, stretching across the oral 

 cavity from a point near the tip of the now forming lower jaw 

 to a point (pp.} on the dorsal side of the cavity just forwards of 

 the anterior limit of the thalamencephalon where it rests upon 

 the dorsal wall of the foregut. It is thus seen that the position 

 of the membrane relative to surrounding parts has not changed 

 from the preceding stage. The floor of the foregut, however, 

 has dropped down in the anterior part, making a rather deep 

 cavity (fg.) immediately below the thalamencephalon and behind 

 the oral plate. The dimensions of the thalamencephalon have 

 increased vertically and decreased antero-posteriorly. The 

 infundibular fold has apparently widened, but not deepened. 

 The optic chiasma has greatly thickened, while the base of the 



