236 GASTRULA-ARRESTED EMBRYOS 



lister and Moore (1937) for R. sylvatica are usable interchange- 

 ably (Figs. 1 and 2). Of course, they are not alike in all respects. 

 For example, their blastopores are shaped differently, and as 

 larvae they are lordotic to different degrees. Under precisely sim- 

 ilar environmental conditions, furthermore, they may undergo 

 corresponding changes at different rates. A R. pipiens zygote, for 

 instance, will develop into an early gastrula in about 26 hours at 

 18° C, whereas a R. sylvatica zygote will do the same thing in 

 only 19 hours. Also, the temperature ranges within which they 

 develop normally are not the same (Moore, 1939): R. pipiens 

 embryos tolerate any temperature between 6° C and 28° C; 

 R. sylvatica embryos any temperature between 2° C and 24° C. 

 But, generally speaking, our knowledge of the phenotypes of 

 members of the parental species does not suggest explicit hypoth- 

 eses about their genotypes which are presently useful in explain- 

 ing why hybrids develop improperly. 



The basis of present ideas about these hybrids is Moore's care- 

 ful descriptive study ( 1946 ) together with the experimental work 

 stimulated by it. The latter includes Moore's own analyses of the 

 capacities of hybrid tissues to induce secondary embryonic struc- 

 tures in normal embryos ( 1948 ) and to respond to normal em- 

 bryonic inductors (1947), but the subjects of these particular 

 papers fall outside our scope. In addition, Moore (1955) has 

 published a general review of recent work on anuran hybridiza- 

 tion, in one section of which the interested reader will find briefly 

 summarized most of the findings that will now be presented in 

 greater detail. 



Morphogenesis of Hybrid Embryos 

 General 



The following statements are intended to characterize the 



chief, easily observable characteristics of developing hybrids.* 



1. Hybrid embryos cleave and blastulate in the same pattern as 



* By "hybrid" or "hybrid embryo" we shall mean any embryo whose fe- 

 male parent is a member of R. pipiens and whose male parent is a member 

 of R. sylvatica. 



