No. 2.] THE HYPOPHYSIS OF A MI A CALVA. 73 



spicuous," I may say that it arises, as will be shown, at a time 

 intermediate between the time of its first occurrence in the 

 other two forms in which this point has been determined, and 

 it is certainly a conspicuous, I might say, striking differentia- 

 tion of cells, even in its fundaments, although it may not be a 

 prominent object in point of size or in the distinctness with 

 which its limits may be fixed. The actual time of its appear- 

 ance I have found to be several hours earlier than the time 

 assigned by Dr. Dean, as a comparison of Fig. 4 with his 

 Fig. O plainly proves. His figure and description indicate a 

 stage nearly the same as my Fig. 4, which is shortly after the 

 time of hatching. Although in his figure the oral plate is still 

 unbroken, the cavity of the foregut posterior to it has notice- 

 ably deepened by the dropping downwards of its basal wall, 

 while in the stage I figure the cavity is of about the same 

 depth, and the oral plate is only severed at its middle point, 

 remaining intact at the sides. While in Fig. 4 the cells, which 

 he would call the beginning of the hypophysis, may be said to be 

 "continuous with the cells of the formative epiblast," being in 

 the same plane with them, there is no other evidence of a con- 

 nection and certainly no ground whatever for considering them 

 derived th'erefrom, as the enlargement of this section (Fig. 5) 

 makes very clear their differentiation from cells of the hypo- 

 blast. But its earliest stage is found (Figs. 2 and 3) some 

 hours previous to this stage, far back of the oral plate and the 

 epiblast, as a well-defined modification of cells of the basal 

 layer of hypoblast where the post-optical lamina of the brain 

 rests closely upon it. Considering the common assumption of 

 embryologists that its origin is from the epiblast, my observa- 

 tions become of interest, since I believe that I have proved 

 beyond question that the hypophysis is of hypoblastic origin, 

 and I do not at present understand how Dean could have 

 considered it otherwise. 



Balfour and Parker l state in a footnote : " We have not 

 been able to work out the early development of the hypophysis 

 as satisfactorily as we could have wished. . . . Were it not for 



1 " On the Structure and Development of Lepidosteus," Trans. Roy. Soc. 

 Pt. ii, p. 379. 1882. 



