ORGANS IN THE EMBRYO OF THE FOWL. 99 



ally. How has the head-fold been formed ? The great expansion 

 of the body cavity (amniocardiac vesicles) on each side causes 

 an elevation of the anterior angle of the ectamnion and a pocket 

 is formed by fusion of its opposite limbs, which have a strong af- 

 finity for each other ; fusion proceeds along the median dorsal 

 line so long as the energy of fusion is sufficient to draw the so- 

 matopleure up. The head of the embryo is rapidly elongating at 

 this time and slips into the pocket thus formed, being guided in 

 part by the cranial flexure (Fig. i). It is interesting to note how 

 far the ectodermal thickening stretches ahead of the mesoderm of 

 the fold near the point of closure, and that the apical cells are 

 elongated into pseudopodium-like processes. 



The histological differentiation of the amniotic area of the 

 somatopleure from the chorionic portion precedes the elevation 

 of the fold. 



This brief inquiry, then, suggests that the order of events in 

 the formation of the head fold of the amnion is : 



1. Thickening of the ectoderm on the outer margin of the am- 

 niogenous somatopleure, beginning in -front of the head of the 

 embryo and extending back on each side (ectamnion). 



2. Great expansion of the body cavity on each side opposite 

 the head of the embryo and consequent elevation of the anterior 

 bay of the ectamnion to the level of the dorsal surface of the 

 embryo. 



3. Fusion of the right and left limbs of the ectamnion, begin- 

 ning at the angle, to form a pocket, the head-fold of the amnion. 



4. Pushing of the head of the embryo into the fold. 



There may be, however, considerable variation in the time of 

 formation of the head-fold. I have, for instance, one series with 

 17-18 mesoblastic somites (ser. 175), where the head-fold is not 

 yet formed. 



Extension of tJic Ectamnion. The ectamnion differentiates 

 backward more rapidly than the lateral folds, and always pre- 

 cedes their origin. In the 48-hour stage (2122 somites) (Fig. 

 3) the ectamnion from in front has joined that from behind 

 formed in connection with the tail-fold. There is a place, corre- 

 sponding nearly to the final meeting place of anterior and poste- 

 rior lateral folds, where it becomes very faint. It would appear 



