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Show 1894.] TADPOLE OP XENOPUS LJEVIS. 107 directed processes from the first and second branchial arches ; are somewhat conical in shape, with rounded or very slightly notched hinder borders." This description applies in the main to tadpoles of Xenopus of M a y 31st. The opercular fold is then only commencing to grow, and processes from the three first branchial arches just project beyond the line of the body. In the lax tissue lying in the interior of these processes is a capillary vessel derived from the vascular arch. The processes, however, are hardly conical in form; they have a long base of attachment, and are indeed rather to be described as lamellae than processes. Pronephros.-I carefully investigated the pronephros, but with entirely negative results so far as the discovery of anything of novelty is concerned. It is precisely like that of Rana, and opens into the body-cavity by three funnels opposite to its glomerulus. Vascular System.-Messrs. Marshall and Bles have described with such miuuteness the development of the heart and arterial system in Rana temporaria that a comparison with the corresponding stages of Xenopus becomes easy. It is very remarkable, as they point out, that the condition of the vascular arches should differ so much from that of the closely-allied Rana esculenta. In the latter, according to Maurer (quoted by Messrs. Marshall and Bles), the afferent and efferent branchial vessels are continuous with each other, formiug complete arches. In one specimen of Rana temporaria the same continuity was noted, but as a rule the communication between afferent and efferent sections of the aortic arches was indirect through the branchial capillaries. In view of this difference between two species of one genus, the fact that Xenopus agrees with Rana esculenta is of less interest. In Xenopus it is quite easy to trace the four aortic arches from the heart to the dorsal aortae. The truncus arteriosus first divides into two branches (on each side); the posterior of these again divides into two, and a little later the vessel which is now the hindermost itself divides into two trunks ; thus the four afferent branchial vessels arise. Messrs. Marshall and Bles figure (he. cit. pi. xiv. fig. 6, A H ) a short diverticulum of the truncus arteriosus lying in front of the fully-developed first branchial arch in tadpoles of 5 millim.; this they consider to be referable to the hyoid arch. It disappears soon. I find an entirely similar diverticulum of the first arch in Xenopus in a tadpole of 7 millim.; it was present on both sides of the body. In tadpoles of June 2nd there were only three vascular arches. The fourth arch, arising from the third, went straight to the lung. EXPLANATION OF PLATE XIII. Fig. 1. Tadpole of Xenopus Icsvis of June 5th. Figs. 2, 3. Dorsal and ventral views of an older tadpole. Figs. 4, 5. Lateral and dorsal views of a full-sized tadpole. |
