268 INEZ L. WHIPPLE. 



alpestris, length from tip of snout to cloaca, 13 mm.; (3) small 

 specimens of Diemyc tyliis viridcsccns in the terrestrial stage, 32 

 68 mm. in length ; (4) larvae of Spclerpcs rnber, and Spelerpes 

 bilineatns ; (5) larvae of Desmognatlms fnsca from 17.525 mm. 

 in length. 



The methods employed in the study of these larval forms were 

 (i) dissection of the larger ones, prolonged staining in methylene 

 blue being used in some cases to bring out the cartilage ; (2) 

 staining in borax carmine and clearing in toto the ventral wall of 

 the posterior part of the body cavity including the pubo-ischium 

 and proximal portion of the femur ; (3) horizontal serial sections 

 of the ventral body wall ; and (4) transverse serial sections of the 

 posterior part of the body. In the case of each of the younger 

 stages all three of the latter methods were used. 



In 37 and 42 mm. long Amblystoma opacum larvae there is no 

 trace of the ypsiloid cartilage. The two halves of the pubo- 

 ischium are quite separate. The muscular abdominal walls show 

 the two primitive laminae (obliquus externus profundus and ob- 

 liquus interims) with the rectus abdominis as a ventral continua- 

 tion of both. In the larger specimens the obliquus externus 

 superficialis and the transversalis appear as secondary develop- 

 ments in the form of very thin laminae. There is a noticeable 

 difference in size of fiber between the medial well differentiated 

 portion of the rectus abdominis and the more lateral region 

 which grades imperceptibly into the deep external oblique on the 

 outside and the internal oblique within. The latter region of the 

 rectus abdominis (/. e., the rectus abdominis profundus), like the 

 two primitive laminae with which it is continuous, consists of 

 large fibers two or three times the diameter of those constituting 

 the medial portion of the rectus. The latter are thus easily 

 recognized both in sections and in the in toto preparations. 



All of these early stages show that in the somite immediately 

 anterior to the pubo-ischium, the inner portion of the rectus 

 abdominis is differentiated into a muscle which deviates sharply 

 from the general longitudinal course of the rectus abdominis. 

 Its fibers, which are small like the rest of the medial portion of 

 the rectus abdominis, arise in the lateral processes and along the 

 anterior edge of the pubis and extend obliquely medially to be 



