ORGANS IN THE EMBRYO OF THE FOWL. 



97 



name ectamnion, precedes by a little the formation of amnioge- 

 nous folds in any region, and indeed it induces the origin of the 

 entire system of folds. It has been described by many embryol- 

 ogists at the stages immediately preceding fusion of the limbs of 



e.a. 



FIG. I. Embryo of chick with 13 mesoblastic somites. University of Chicago 

 Embryological Collection, No. 555. e.a., ectamnion; a.c., inner margin of amnio- 

 cardiac vesicles ; A, region of the somatopleure destined to form the body-wall ; B, 

 amniogenous somatopleure ; (7, choriogenous somatopleure. 



the amnion (cf. Schenk, '/i), and it forms the ectodermal sero- 

 amniotic connection of Hirota ('94). But no one, so far as I 

 know, has traced it back to its origin and recognized the fact 

 that it is the earliest formed part of the amnion, which is thus 

 primarily ectodermal in the chick, as in Chelonia and some other 

 primitive Sauropsida. 



The ectamnion may first be distinguished at about the stage 

 with nine mesoblastic somities, where it appears as a median thick- 

 ening of the ectoderm in front of the head near the anterior 

 boundary of the proamnion. Along the line of this thickening 

 there is a fusion, between ectoderm and entoderm. The thick- 

 ening is extended right and left and turns backwards along 

 opposite sides of the head to about the region of the middle of 



