THE YPSILOID APPARATUS OF URODELES. 269 



inserted partly into the linea alba of the first somite. From the 

 point where the first myocomma joins the linea alba, the insertion 

 of the muscle follows an outwardly curving line which ends 

 about half way across the second myotome, and thus with the 

 insertion of the corresponding muscle of the opposite side maps 

 out the exact location of the future Cartilago ypsiloides. This 

 muscle is evidently the M. ypsiloideus posterior and is at this stage 

 the only definite indication of an ypsiloid apparatus. 



In Ainblystouia opacmn larva; of 50 mm. I find that the 

 ypsiloid cartilage has appeared. It possesses practically the 

 adult form and relationship, but is very thin, especially toward 

 the ends of the lateral arms. The stem of the cartilage is quite 

 separate from the pubo-ischium which at this stage still consists 

 of two wholly separate lateral halves. I was unable to obtain 

 larvse of Aniblystoma opacitm between 42 and 50 mm. in length 

 and can therefore make no statement concerning the earliest 

 appearance of the ypsiloid cartilage. Its entire absence in the 

 42 mm. stage, however, considered in connection with its com- 

 plete formation in the 50 mm. stage in which the two halves of 

 the girdle are still separate, points conclusively to the origin of 

 the ypsiloid cartilage independently of the pelvic girdle as a 

 chondrification of the linea alba of the first somite and of the 

 deeper portion of the first myocomma. The possibility of such 

 chondrification of regions of muscular attachments upon which a 

 special strain is brought is well established, and the lack of 

 correspondence in this case between the transverse direction of 

 the myocomma and the curved form of the arms of the cartilage 

 may be looked upon as expressing a resultant of forces, since the 

 arms of the cartilage tend somewhat to follow the direction of 

 the edge of the aponeurosis of the transversalis, the posterior 

 portion of which, we have already seen, is usually eventually 

 inserted into it. 



The horizontal series of sections of the Triton alpestris larva 

 show a well developed ypsiloid cartilage. This is wholly sepa- 

 rate from the pubo-ischium though, as in Amblystoma, articu- 

 lated with it. The two halves of the pubo-ischium are at this 

 stage still quite separate. 



All of the specimens of terrestrial Dicmyctylns which I dis- 



