GENERAL ZOOLOGY 



Dorsal 

 lip 



Dorsal lip 



Blastopore 

 Yolk-laden cells 



Plane of section 

 shown in C 



Fig. 5.29. Diagram showing results of the transplantation of a piece of dorsal lip of the blasto- 

 pore of an amphibian (shown in A) to the blastocoel of another embryo of the same age (shown 

 in section in B). In D, the lower individual has developed under the influence of the grafted 

 dorsal lip; the line c-c' indicates the plane of the cut surface shown in C. In C, cells 

 derived from the graft are shown in black; induced cells are in white. (Redrawn from Analysis 

 of Development, edited by B. H. Willier, P. A. Weiss, and V. Hamburger, copyright 1955 by 

 W. B. Saunders Co., printed by permission.) 



is thrown out of its usual position, the course of development is not normal. 

 This localization of cytoplasmic areas occurs more slowly in some animals 

 than in others. If the first two cells of the frog embryo are separated, each 

 gives rise to a whole embryo; but if one of the cells is injured, the other forms 

 half an embryo (Fig. 5.28). In some other animals each of the first two cells 

 will form only half an embryo if separated. 



As development progresses, certain cell masses are localized and diflferentiate 

 into specific organs. That this differentiation is determined in part by the 

 relationship of the cell mass to its neighboring cells can be demonstrated by 

 changing the typical relationships of the cells. The ectoderm on the ventral 



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