86 EDWARD MCCRADY, JR. 



In the early part of the ninth day (stage 24) the embry- 

 onic head begins to grow forward beyond the coelomic ellipse 

 so that it comes to overlie a portion of the area vasculosa, and 

 simultaneously the mesoderm in this region (i.e., the floor of 

 the subcephalic pocket) becomes resorbed. The portion of the 

 yolk sac which is thus inside the area vasculosa but devoid 

 of mesoderm (the proamnion) is the portion into which the 

 head dips when the cephalic flexure forms. The layer of 

 ectoderm and the layer of endoderm which thus come to 

 enclose the head constitute the head fold of the amnion. 

 Laterally and posteriorly the embryo is still surrounded by 

 the coelomic ellipse, but anteriorly it reaches over the coelomic 

 region and dives head first into a pocket in the yolk sac. 



In the opossum, and generally in the Sauropsida, the 

 cephalic flexure seems to play a mechanical part in making the 

 head fold form before the tail fold, but this is possible only 

 where amniogenesis begins at a late stage of development. 

 In mammals the general tendency is for the amnion to form 

 at -an early stage, and 1) to dispense with folds altogether 

 (as in man where it forms at a stage corresponding approxi- 

 mately to the opossum stage 14) ; or 2) to form from folds 

 simultaneously on all sides (as in the pig where the time 

 corresponds approximately to the opossum stage 21 or 22) ; 

 or 3) to be slightly more advanced at the caudal fold (as in 

 the rabbit where the time is very nearly the same as in the 

 opossum). The opossum thus conforms to the reptilian or 

 avian type in time of amnion formation and in precocity of 

 the head fold, but probably these relationships are not very 

 significant. 



Selenka (loc. cit.) was the first to point out the unique 

 constitution of the head fold of the amnion in the opossum. 

 The fact that the proamnion, which is very transitory in other 

 animals, is persistent in the opossum leads to a very com- 

 plicated situation when all the amniotic folds meet during 

 stage 29. 



Fusion of the hearts in the ventricular region. In stage 25 

 the concrescence of the lateral body walls in the head region 



