554 BERTRAM G. SMITH 



non-placental forms a large amount of yolk is certainly necessary , 

 for the food (e.g., insect larvae) of the young is larger and less 

 easy to capture than is the case with aquatic larvae. On the 

 other hand a large store of yolk is common in the eggs of fishes. 

 The most we can infer is that the presence of a large amount 

 of yolk in the egg is one of the conditions that makes possible 

 the invasion of the land; it cannot be said that the presence of 

 an unusual amount of yolk in the egg of Cryptobranchus is 

 evidence either for or against a terrestrial ancestry. 



Among the known eggs of urodeles that of Cryptobranchus 

 is probably the most heavily yolk-laden; the egg of Necturus 

 contains nearly as much yolk. In the salamandrina one notes 

 the large amount of yolk in the eggs of Desmognathus (Wilder 

 '04; Hilton '04 and '09); Spelerpes (Goodale '11) and Plethodon 

 (Piersol '09). 



The absence of pigment in the egg of Cryptobranchus is cor- 

 related with the nesting habits, whereby the eggs are protected 

 from the light. Similar conditions are found in Necturus, Des- 

 mognathus, Plethodon and Spelerpes. 



The occurrence of a protoplasmic mantle and cytodisc in the 

 late ovarian egg of Cryptobranchus parallels a condition in the 

 teleost egg which reaches its full expression just before fertiliza- 

 tion; in Cryptobranchus this condition is transient. The segre- 

 gation of a definite layer of cytoplasm close to the surface of the 

 blastodisc in Cryptobranchus shortly after fertilization suggests 

 a parallel with the marked increase in thickness of the germinal 

 disc of the teleost egg immediately after fertilization; Professor 

 Dean informs me that he has observed a similar phenomenon 

 in ganoids. 



The fundamental features of the early embryonic development 

 have to do with the building up of the very general structures 

 and body relations common to all vertebrates. In their most 

 general aspects cleavage and gastrulation are extremely palin- 

 genetic phases of development. But we find secondarily imposed 

 on the essential features of these processes, modifications which 

 are highly coenogenetic and of adaptive significance mainly 

 for the embryo and larva (Lillie '98), rather than for the adult. 



