MORPHOGENESIS AND METABOLISM OF 

 GASTRULA-ARRESTED EMBRYOS 

 IN THE HYBRID 



Rana pipiens ? % Rana sylvatica ^ 



JOHN R. GREGG: zoology department, 



COLUMBIA UNIVERSITY, NEW YORK 



The purpose of the account following is to provide a fairly 

 critical and detailed summary of some recent descriptive and 

 experimental work on embryos belonging to the hybrid Rana 

 pipiens 5 x Rana sylvatica S . Embryos of this type become gas- 

 trula-arrested; that is to say, when they are of gastrula age the 

 morphogenetic movements exhibited by their tissues are not 

 correlated in the normal (R. pipiens) pattern, and to casual ob- 

 servation they appear to live out their lives in a state no more 

 developmentally advanced than that of very young R. pipiens 

 gastrula. 



The fact that such embryos are hybrids, each developing from 

 a zygote formed by the union of an ovum from a member of R. 

 pipiens with a spermatozoon from a member of R. sylvatica, is 

 not yet of much explanatory value in accounting for their defec- 

 tions as developmental systems. Embryos in the two parental 

 species are morphologically similar to a very marked degree. 

 They are of about the same size, they are telolecithal to about the 

 same moderate degree, they cleave totally and in the same pattern, 

 their cells are believed to contain the same number of chromo- 

 somes ( N = 13 ) , and, in general, the sequences of developmental 

 transformations they exhibit are alike to such an extent that 

 Shumway's (1940) normal table for R. pipiens and that of Pol- 



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