351 



adduced by these authors and pointed out the objections to considering 

 them as illustrative of normal developmont. Morgan i) has more 

 recently pointed out others. 



The observations of von Baer, Reichert, Newport, PflIIger 

 and others foreshadowed the inadequacy of the theory when applied 

 to the Amphibia, while the evidence adduced by 0. Schultze, Assheton, 

 H. V. Wilson and myself have shown that it is no longer tenable. 



In the accompanying diagrams (Figs. 28 — 31) I have endeav- 

 ored to point out some of the more important changes which have 

 been brought about in 



the formation of the ^8 ^^ 



embryo through a con- 

 stantly increasing 

 amount of yolk ma- 

 terial as compared 

 with the amount of 

 germinal material. In 

 general the views are 

 those expressed in a 

 previous paper. The 

 experiments on Nec- 

 turus, however, lead 

 me to qualify some 

 whatmy previous sta- 

 tements regarding the 

 formation of the Am- 

 phibian embryo. 



Fig. 28 represents the conditions observed in Rana, Bufo, Acris 

 and Chorophilus. The blastodisc, represented by the dotted area, has 

 extended well over the yolk, reaching the line B. I. before the body 

 of the embryo is differentiated. When the embryo is first discernable it 

 extends from Ä to B. as conclusively shown from previous experiments. 

 This portion, represented by the broad black line, is differentiated in 

 situ. A small portion of the embryo is formed through a backward 

 extension of the dorsal lip of the blastopore. This I have represented 

 by cross lines. I would not maintain that the relative extent is precisely 

 shown in the diagram, but would thus emphasize the fact that a part 

 of the embryo is formed in this manner. The remainder of the embryo 

 (P. S.) is formed by the concrescence of the marginal thickening of 



1) The Formation of the Fish Embryo. Journ. Morphol. Boston, 

 Vol. 10, 1895, p. 445. 



.-PS. 



