238 GASTRULA-ARRESTED EMBRYOS 



should be noted that not all gastrula-arrested anuran hybrids 

 survive this long. Embryos belonging to the hybrid Ranu sylva- 

 tica 2 X Rana pipiens S , for example, cytolyze relatively soon 

 after the onset of gastrulation, when their controls are in Stage 

 13. Therefore, hybrids of the sort with which we are concerned 

 in this report are exceptionally favorable experimental material. 



a 



n 



o 



O 



w^ 



Wo 80 120 160 200 



HOURS DEVELOPMENT 18®C 



Fig. 3. Development of embryos in the hybrid Rana pipiens 9 x Rana 

 stjlvatica $. (Reconstructed from data of Moore, 1946.) 



Figure 3 is a schematic representation of the developmental 

 events just outlined. 



Morphogenetic Movements 



Unfortunately, we are not in possession of an adequate gen- 

 eral theory of amphibian gastrulation in terms of which we can 

 explain hybrid developmental patterns. Nor, for that matter, do 

 we have precise ways even of describing various tissue move- 

 ments that occur when embryos gastrulate. Nevertheless, gas- 

 trulating amphibians all exhibit movements belonging to some 

 roughly identifiable types. The classification following is useful 

 for our present purpose. A more exhaustive one is given by Nel- 

 son (1953). 



