EMBRYOLOGY OF CRYPTOBRANCHUS 



535 



198 199 



Figs. 198 Postero-dorsal view of a gastrula of Cryptobranchus allegheniensis 

 with an unusually large yolk plug. The blastopore extends around the egg as 

 a complete circle, but only the dorsal portion shows distinctly in the photograph. 

 From preserved material. X 4. 



Fig. 199 Posterior view of an embryo shortly before the closure of the neural 

 folds, showing persistent yolk plug. Photographed from preserved material. 

 X4. 



2. Exovate abnormality 



A rather common abnormality originating at the time of the 

 formation of the neural groove may be called the exovate abnor- 

 mality. A small protuberance on the site of the closed fenestra 

 takes the form of a spherical exovate; in some cases this reaches 

 a diameter about half as great as that of the egg. In most cases 

 the extra-ovate remains connected with the egg by a very narrow 

 stalk; the malformation is entirely extra-embryonic, the pro- 

 truded yolk is gradually absorbed and the egg produces a normal 

 larva. In other cases the protuberance takes the form of a dome- 

 like swelling and increases in size until the egg collapses. 



3. A double embryo 



On September 27, 1906, I found a nest containing embryos 

 in an advanced stage of development, and among them the double 



