I2O FRANK R. LILLIE. 



tion. The correlations in the development of the amnion are of 

 three kinds : 



1 . Mechanical. Under this head I class the elevation of the 

 lateral amniotic folds, which takes place only after the establish- 

 ment of the head-fold, and which is omitted, if for any reason the 

 head- fold fails to appear or is destroyed. 



2. TropJiic Stimulation. Under this head I class the influence 

 of the traction exerted by the union of the right and left amniotic 

 folds on the amniogenous somatopleure, and the influence of the 

 turning of the embryo on the amniogenous somatopleure of the 

 left side. The influence of the traction in either case is to in- 

 crease the extent of the amniogenous somatopleure, in part (pre- 

 sumably) by stimulating its growth, in part undoubtedly by mere 

 stretching. If, owing to failure of formation of the head-fold, 

 such traction is not exerted on the somatopleure it does not ex- 

 pand nearly to the normal extent. 



3. Differential Stimulation. Under this head I class (doubt- 

 fully) the propagation of the ectamniotic thickening along the 

 somatopleure ; though this may be a process of self-differen- 

 tiation. 



Self -differentiation of the Formation of the Amnion. The for- 

 mation of the original rudiments of the ectamnion may be a 

 process of self-differentiation, though the definite relation of the 

 anterior and posterior rudiments to the head and tail respectively 

 suggests correlation with their formation. Moreover, a slight 

 histological differentiation appears between the amniogenous and 

 choriogenous somatopleure, before, and even in the absence of, the 

 formation of folds, which is apparently not correlated with any 

 other of the processes observed. 



Beyond this mere classification I do not desire to go at present, 

 but will reserve a general discussion of principles until the com- 

 pletion of other parts of the present series. 



In conclusion I simply summarize the results : 



I. MorpJwlogical. 



i. The amnion is primarily an organ of the ectoderm in the 

 chick. The ectamnion first forms in front of the head and dif- 

 ferentiates progressively backwards towards the posterior end, 



