THE NESTS AND LARVJE OF NECTURUS. 195 



free as a larva, the animal is really in an embryonic state so far 

 as its food supply is concerned, until long after its liberation. 

 The only advantages secured by hatching in this condition would 

 seem to be better aeration and opportunities for exercise; these 

 advantages must be in part offset by the increased danger of 

 capture by some larger animal. 



Aside from differences in the stage of development, the em- 

 bryos of Necturus and Cryptobranchus at about the time of hatch- 

 ing are so much alike that it would be difficult to tell them apart 

 were it not for the noticeably greater bulk of the latter. At the 



FIG. 3. Necturus larva reared from advanced embryo obtained from Lake Mon- 

 ona, and preserved in formalin July 31, 1910. (X 



time of hatching the Necturus embryo measures about 18 mm. in 

 length, the Cryptobranchus embryo about 24 mm.; this difference 

 may be due in part to the more advanced condition of the latter, 

 but throughout the entire embryonic history the size of embryos 

 of corresponding stages is so much greater in the case of Crypto- 

 branchtis that though eggs from different nests of the same 

 species may vary slightly in size, the extremes of variation of the 

 two species do not overlap. 



2. Larval Development. I have examined a series of early larval 

 stages kindly loaned me for the purpose by Dr. Bennet M. Allen. 

 The series comprises twelve specimens raised from advanced 

 embryos obtained from Lake Monona and preserved in formalin 

 on the following dates: July 31, August 19, 21, 25, 1910. 



