FROM TWELVE TO THIRTY-SIX SOMITES 



139 



e.a. 



depression bounded in front by the ectamnion, and on the sides 

 by the amnio-cardiac vesicles of the body-cavity (Fig. 65). The 

 floor of the depression is the proamnion. Just before the for- 

 mation of the head-fold proper, the ectamnion in front of the 

 head becomes irregularly thickened to 

 such an extent as sometimes to present 

 an actually villous surface (Fig. 77; cf. 

 Fig. 67). 



The head-fold of the amnion begins 

 to form at about the same time as the 

 cephalic flexure. The great expansion 

 of the body-cavity on each side of the 

 head (amnio-cardiac vesicles) causes an 

 elevation of the anterior angle of the 

 ectamnion, and a pocket is formed by 

 fusion of its lateral limbs. This slips 

 over the head of the embryo with aid 

 of the ventral flexure of the head just 

 developing. Inasmuch as the anterior 

 angle of the ectamnion is in the pro- 

 amnion, where there is no mesoderm, 

 and where the ectoderm is in immediate 

 contact with the entoderm, the ento- 

 derm as well as the ectoderm of the pro- 

 amnion is drawn into the head-fold, so 

 that the latter is not at first a fold of 



Fig. 76. — Entire embryo 

 of 13 s, to show the rela- 

 tions of the ectamnion. 



a. c. Inner margin of 



the somatopleure. But in the chick the amnio-cardiac vesicles. 



proamniotic part of the head-fold is e. a Ectamnion 



i^ ^ A. Region of the soma- 



never very extensive and does not at any topleure destined to form 



time extend back of the beginning of the body-wall. 



. B. Amniogenous soma- 



the mid-brain. Moreover, it is soon m- topleure. 



vaded (Fig. 75) by the body-cavity, and ^^ ^^^^^''^'^''^^^'''^^ ^°"'^" 

 then the entoderm is withdrawn and 



becomes part of the general splanchnopleure. The proamnion 

 ventral to the head is not invaded by mesoderm until a much 

 later period. 



The ectodermal thickening marking the junction of amniotic 

 and chorionic somatopleure extends backwards very rapidly and 

 always precedes the origin of folds in any region. The lateral 

 folds themselves appear to owe their origin to the progressive 



