60 P RATHER. [VOL. I. 



hatching, as shown in Figs. 2 and 3. In order to show the 

 relations of position which the various organs bear to one 

 another prior to the differentiation of the hypophysis, I figure 

 a median sagittal section of an embryo surrounding about 245 

 of the circumference of the yolk, corresponding to an age of 

 about 148 hours (Fig. i). At this stage the foregut (fg.) is 

 seen to have formed as far back as the posterior limit of the 

 third primary vesicle {/ib.}, where the endoderm forming its 

 wall is reflexed upon itself and passes forwards again over the 

 surface of the yolk (}>.). By tracing the foregut through the 

 successive sections of the series, we find it to be a very broad 

 cavity compressed dorso-ventrally until its upper and lower 

 walls are in close apposition in the region immediately under 

 the base of the first primary vesicle, which rests directly upon 

 its dorsal wall. Its walls are slightly separated both anterior 

 and posterior to this region. In the median plane, as shown 

 by the figure, the cavity can be traced forwards to a point 

 somewhat anterior to the front wall of the brain (br.). From 

 its anterior end a diverticulum may be traced on either side 

 in front of the brain, nearly to the dorsal median plane. These 

 endodermal diverticula later become transformed into the larval 

 adhesive organ, as has recently been shown. The endodermic 

 layer increases in thickness anteriorly until a maximum thick- 

 ness is attained in the adhesive organ just mentioned. The 

 walls of the endoderm cells cannot be distinguished, owing to 

 the great amount of yolk material found in them. 



The ectoderm at the anterior end is invaginated in two 

 places. The upper invagination (o.) is merely a depression 

 within the adhesive organ which is developing beneath the 

 ectoderm by diverticula from the foregut, as just described, 

 pushing the ectoderm outwards in the form of a circular ridge 

 (ao.) around the snout. The lower invagination is the involu- 

 tion for the stomodaeum (.$/.), and has already pushed inwards 

 nearly to the endoderm surrounding the foregut. This invagi- 

 nation is on a lower plane than the foregut and is directed dis- 

 tinctly downwards, making a small angle with that plane. 



The large cavity (c.) between the brain and the epiblast in 

 front is for the most part filled by the developing adhesive 



