366 EYCLESHYMER. [Vol. X. 



that from eggs manifesting such diversities of cleavage iden- 

 tical embryos, so far as we are able to determine, arise. If 

 abnormal embryos resulted, the variations from a given type 

 might bear a direct causal relation ; but from the evidence 

 bearing upon this point such does not appear to be the case, 

 and my study only emphasizes the truth of the statement made 

 by Jordan and myself ('9i) that " the irregularities of cleavage 

 have no appreciable effect upon any stage of development of 

 the embryo." 



The above being true, two alternatives present themselves. 

 Either the q.^% is isotropic, or its mosaic character is not 

 revealed by its mode of cleavage. If the former be true, we 

 are wholly unable to explain the many facts of precocious 

 differentiation so obvious in the Annelids, Molluscs, and other 

 invertebrates. To accept the latter is to admit the inadequacy 

 of the mosaic theory as based upon cleavage phenomena. 



II. Gastrulation to Closure of Neural Folds. 

 I. Formation and Closure of the Blastopore in Amblystoma. 



About 60 hrs. after the first cleavage gastrulation begins. 

 In order to follow the process in detail the following methods 

 were employed : The &gg, while still within its membranes, 

 was placed upon a glass slide, and after allowing the mem- 

 branes to dry just enough to hold the %^'g firmly in situ, the 

 slide was inverted and placed over a moist chamber. This 

 method together with the assistance of a bed of cotton upon 

 which the eggs, with envelopes removed, could be easily held 

 in position, have enabled me to observe the movement of 

 individual cells. 



At the close of cleavage there is an irregular broken line 

 (pp., PI. XX, Fig. i) lying just below the equator and formed 

 by the union of a number of cleavage furrows; along this line 

 the process first begins. While in Amblystoma invagination 

 always begins nearer the equator than the vegetative pole, 

 other conditions have been observed in Amphibia. In Rana 

 Pfliiger ('83) and Roux ('88a) find the blastopore first appearing 

 at the equator of the ^gg, while Houssay ('9o) in the Axolotl, 



