Development of the Third Day 189 



enteron (Fig. 49, C). During the third day 

 the heart has shifted its position so far to the 

 rear that there is a distinct space between it 

 and the now more sharply defined head (Fig. 

 57) : this space may be called the neck; in this 

 region there has been no cleavage of the 

 mesoblast, so that the three layers, the ento- 

 blast, mesoblast, and ectoblast form one con- 

 tinuous layer of tissue as we pass outwards 

 from the fore-gut or pharynx to the exterior. 



The entobiastic lining of the pharynx, 

 during the latter part of the second or early 

 part of the third day, becomes pushed out, 

 on each side, as four narrow pouches, the 

 visceral or gill pouches, similar to the five 

 pouches that have been described (pages 

 51 and 52) in connection with the frog. 



The first three of these pouches, during 

 the third and fourth days, open to the 

 exterior, their entobiastic walls fusing with 

 the surface ectoblast (Fig. 63, fb). Each 

 gill, branchial or visceral cleft, as it is vari- 

 ously called, is an actual, narrow chink opening 

 from the anterior end of the digestive tract 

 to the exterior. Owing to the curvature of 

 the neck, the clefts are not parallel to each 

 other, but converge slightly towards a point 



