336 The Allirator and Its Allies 



The medullary folds have a curious origin, 

 difficult to explain without the use of figures. 

 They are continuous posteriorly with the primitive 

 streak, so that it is impossible to tell where the 

 medullary groove ends and the primitive groove 

 begins, unless the dorsal opening of the blastopore 

 be taken as the dividing point. 



The amnion develops rapidly, and entirely from 

 the anterior end. 



The blastopore or neurenteric canal is a very 

 distinct feature of all the earlier stages up to about 

 the time of closure of the medullary canal. 



Preceding the ordinary cranial flexure there is a 

 sort of temporary bending of the head region, due 

 apparently to the formation of the head-fold. 



During the earlier stages of development the 

 anterior end of the embryo is pushed under the 

 surface of the blastoderm, and is hence not seen 

 from above. 



Body torsion is not so definite in direction as in 

 the chick, some embryos lying on the right side, 

 others on the left. 



Of the gill clefts, three clearly open to the exte- 

 rior and probably a fourth also. A probable fifth 

 cleft was seen in sections and in one surface 

 view. 



The first trace of the urinary system is seen as a 

 dorsally projecting, solid ridge of mesoblast in the 

 middle region of the embryo, which ridge soon 

 becomes hollowed out to form the Wolffian duct. 



