No. 2.] THE HYPOPHYSIS OF AMIA CALVA. 6 1 



organ. But surrounding this may be seen mesodermal cells 

 which form a strand running downwards between the stomo- 

 daeum and foregut and connecting with those forming the 

 heart (/i.) below. The chorda extends no further forwards than 

 the posterior margin of the hind brain. The brain is not as 

 clearly delimited from the ectoderm above as the figure indi- 

 cates. The first primary vesicle (/?>.) is evaginated at its base 

 both in front and behind, giving rise to the recessus opticus 

 (ro.} and the infundibulum (/;/.) respectively. The base of this 

 vesicle is arched considerably and conforms closely to the dorsal 

 wall of the foregut. 



In an embryo about 160 hours old (Fig. 2) marked changes 

 may be noted in all the organs described in the preceding stage. 

 The adhesive organ has broken through the ectoderm, forming 

 a semicircular row of sucking cups on either side of the snout. 

 The section, not being exactly vertical, passes through one of 

 these cups (sc.) on the dorsal side. The space between this 

 organ and the brain is now filled with mesoblastic cells (ms.). 

 The stomodaeal invagination has deepened a little, owing to 

 the development of the sucking disc over and in front of it. 

 By the vertical growth of the brain the anterior end of the 

 alimentary canal has been pushed downwards to a level with 

 the mouth fold, so that the ectoderm lining the stomodaeum is 

 in close contact with the endoderm forming the wall of the 

 foregut. The oral plate (op.} thus formed is on the point of 

 breaking through, and the point of fusion of the endoderm roof- 

 ing the gut with the ectodermal roof of the stomodaeum is 

 scarcely recognizable. There is no fold of the ectoderm com- 

 parable to " Rathke's pocket " discernible, nor is there an endo- 

 dermal fold comparable to " Seesel's pocket." The ectoderm 

 immediately over the stomodaeum is much thickened and com- 

 posed of large cells of irregular shape loosely aggregated. 



Fig. 3, an enlarged portion of the same section, will show 

 that this ectodermal layer (cc.} terminates rather abruptly at 

 the point of junction (//.) with the endoderm (en.}, at which 

 point the cells are seen suddenly to become smaller. While 

 they are somewhat disconnected in this region, they soon become 

 arranged into three definite layers running back under the ante- 



