258 INEZ L. WHIFFLE. 



PART I. ANATOMY OF THE YPSILOID REGION IN URODELES. 

 A. Description of Adult Forms. 



i . Diemyctylus viridescens. In Dicmyctyhis, as in all Urodelcs, 

 the muscular abdominal walls are differentiated into the four typi- 

 cal layers, viz., Mnsculus obliqnus extermts, M. obliqims interims, M. 

 rcctus abdominis and M. transversalis. The external oblique is 

 strongly developed, showing the typical outer and inner laminae. 

 The outer has lost its metamerism except in its origin ; its fibers, 

 which form a strong, continuous, muscular sheet, are mainly in- 

 serted into a wide, ventral aponeurosis, and the posterior bundles 

 into ilium and pubis. The rectus abdominis consists of a narrow 

 band of muscle, lying on either side of and contiguous to the linea 

 alba and covered ventrally by the aponeurosis of the obliquus 

 externus. It is very primitive in character, consisting of a series 

 of myotomes, separated by well formed myocommata. The 

 obliquus internus is almost vestigial and in some individuals I was 

 unable to find it. Its very thin layers of fibers arise mainly by 

 digitations from myocommata beneath the deeper layer of the ex- 

 ternal oblique, although the more posterior bundles have their 

 origin in the anterior edge of ilium and pubis. The fibers of this 

 muscle extend very obliquely, anteriorly and ventrally and are 

 inserted into myocommata. At the edge of the rectus abdomi- 

 nis muscle the fibers of the internal oblique seem in some cases 

 to become continuous with those of the rectus, in other cases to 

 pass dorsal to the rectus, which, however, they only slighly 

 overlap. 



The transversalis, even more than the obliquus externus, con- 

 sists of a continuous muscular sheet, its metamerism being evident 

 only in its origin. It is inserted ventrally into an aponeurosis 

 which is somewhat narrower than that of the obliquus externus 

 and which lies on the inner (7. <\, dorsal) surface of the rectus ab- 

 dominis. This aponeurosis narrows abruptly in the two meta- 

 meres anterior to the pubis, and ends at the pubic symphysis in a 

 point. 



In the two myotomes immediately anterior to the pelvic girdle 

 there is considerable muscular differentiation, which may be most 

 easily shown by a series of dissections. Fig. I shows a ventral 



