ORGANS IN THE EMBRYO OF THE FOWL. 121 



where it is met by the posterior ectamnion differentiating for- 

 wards. Thus the amniotic zone of the somatopleure is marked 

 off from the chorionic zone. 



2. The head-fold is formed from the ectamnion with the coop- 

 eration of the amnio-cardiac vesicles and of the proamnion which 

 is depressed between the former. The immediate prolongation 

 of the head-fold is produced by the progressive fusion of the 

 ectamniotic rudiments backwards, and it includes only an ex- 

 tremely small part of the proamnion. 



3. The tail-fold is likewise formed primarily by the ectamnion 

 with participation of the amnio-allantoic enlargements of the 

 body-cavity. 



4. There are certain constant secondary folds in the upper 

 (right) limb of the amnion produced by the turning of the em- 

 bryo. These persist longest in the region of the heart and im- 

 mediately behind it. 



1 1 . Experimental. 



1 . Destruction of the anterior ectamniotic rudiment of one side 

 prior to the formation of the head-fold of the amnion results (<?) 

 in permanent absence of the amnion back to the hind-limbs 

 (exp. 57); (b) in inhibition of the growth, and almost complete 

 suppression of the folds of the amniogenous somatopleure of the 

 uninjured side; from which we may conclude - 



2. That the growth of the amniogenous somatopleure is nor- 

 mally induced by the traction exerted on it by the progressive 

 fusion of the folds, and that the uprising of the folds is due to 

 the lifting power of the same process of fusion. 



3. The tail-fold and posterior lateral folds cannot replace the 

 anterior lateral and head-folds, nor can the latter replace the 

 former. 



4. Not only the initiation, but also the progress of the forma- 

 tion of the anterior lateral folds is dependent upon the perfection 

 of the head-fold (exp. 36). 



5. The absence of the amnion has, at least for a time, only a 

 limited effect on the development of the allantois. 



6. Inasmuch as the embryo may develop perfectly normally 

 to the stage of five or six days without the amnion, it is obvious 

 that the functional significance of the latter must be slight during 



