346 n. B. POLLARD. 



rudimentary hyomandibular cleft {H. M., fig. 11). As in the 

 former stage, the lumen of the alimentary canal does not 

 extend further forward than to the level of the hyoid clefts. 

 The T-shaped lumen is constant for about four days of 

 development. 



Fig. 3 is a diagrammatic representation of the conditions at 

 this stage. Ectoderm, endoderm, and mesoderm are tinted, as 

 in fig. 2, and the brain and other organs, which are represented 

 as being solid, divided in the middle line so as to show to some 

 extent the vertical relations. The view is, as in fig. 3, from 

 the ventral side. 



The ingrowth of ectoderm is seen to start from below the 

 recessus opticus, choroid fissure, and posterior region of the 

 eye, and it is now throughout of more uniform thickness than 

 in the earlier stage. As stated above, the ectoderm is more 

 condensed below the infundibulum, and is thicker where it is 

 embraced by the maxillary and mandibular processes. In the 

 figure the maxillary process is indicated by the dotted line. 

 The yellow dotted line represents the rudimentary hyoman- 

 dibular gill-pouch. Ectoderm and endoderm pass into one 

 another without demarcation. 



On the fourth day the hypophysis becomes more rounded, 

 though still more elongated than in its later stages, and at the 

 same time it is becoming cut off from the mouth. The cells 

 of the ectodermic ingrowth which forms the mouth are 

 arranging themselves in layers along the upper and lower jaws, 

 so as to give rise to a central cavity. An epithelium continuous 

 with the epithelium of the hollow alimentary canal becomes 

 difierentiated from behind forwards. In subsequent stages 

 the hypophyis becomes separated from the mouth, and a mem- 

 brane is found to surround it. Thus the condition depicted 

 in the majority of Professor Dohrn's figures is arrived at. 



Mesoderm. 



The origin, extent, and fate of mesodermic structures are by 

 no means easy to determine in Teleostei, on account of the 

 fusions of the three layers and the indefiniteness of the cells. 



