506 PROCEEDIXGS OF THE ACADEMY OF [AugUSt, 



gills aud l)oth pairs of limbs. The sections show that prouephroi 

 are present in the anterior trunk somites, and metanephroi (M. T., 

 fig. 5) in the posterior somites. There is a cartilaginous brain 

 capsule, cartilaginous vertebral arches {V.C., figs. 4 and 5), and 

 cartilages for the bones of the limbs. The notochord {N. C. ) is 

 undergoing degeneration; the somatic mesoderm (*So.J/. ) is 

 divided into its various components — muscle somites, sclerotom, 

 etc. The segmental ducts {S.D.) extend posteriorly and open 

 into the rectum. The coelom ( C) is large, the median* mesenteries 

 well formed (J/.), the liver (L.) aud hepatic ducts present. The 

 epidermis (Ed.) is thick and glandular on the dorsal side of the 

 body, aud the genital ridges (G.R., fig. 5) developed. All these 

 points show an advanced stage of development, Avhich makes it 

 the more remarkable that the middle iutestiue should be represented 

 by a large yolk -sphere. 



The sides aud ventral aspect of the yolk-sphere are covered with 

 a very thin envelope composed of three cell layers closely apposed, 

 the flattened ectoderm (Ect, figs. 4, 5), the somatic mesoderm 

 (So. 31.) and the sphlanchiuic mesoderm (»S/;.Ji. ). The fact that 

 the body wall is excessively thin upon the ventral side of the yolk- 

 sphere, svould show perhaps that this wall had extended around 

 the yolk not until late in the development. The blood-vessels of 

 the yolk-sphere have their position iu the mesodermal layers. 



In conclusion, we find the principal modification of these larval 

 stages to consist in the presence of a large yolk-sphere, which is 

 an integral part of the mid-gut, while the anterior and posterior 

 portions of the gut are tubular. Peculiar also is the great size of 

 the posterior limbs and of the gills (fig. 2), and the continuance 

 within the egg-envelopes after a time v/heu in other Urodelea the 

 larva has emerged from them. All these modifications must be 

 referred to the terrestrial life; and the great size and long continu- 

 ance of the yolk-sphere may l)e accredited the value of a source 

 of nourishment. A life under still dryer surroundings, and 

 longer life within the egg-envelopes necessitating a larger yolk- 

 sphere, might lead to the formation of a yolk-sac in the strict 

 sense by the holoblastic cleavage becoming mesoblastic. The case 

 of Pletliodon cinereus is but another to show how readily develop- 

 mental processes become modified by change of the environment, 

 and how much care must be used iu interpreting them in the 

 search for affinities. 



