S06 Dr Allen Thomson on the Vascular System 



covers the whole of the yolk at a very early period ; but in the 

 frog no new fold of the germinal membrane takes place for the 

 formation of the intestinal tube (Fig. 8, ef). We may therefore 

 regard the embryo of the frog as destitute of any proper intes- 

 tinal appendage or yolk-bag, as the whole of the mucous layer 

 of the germinal membrane is employed in the formation of the 

 rudimentary intestine ; or, perhaps, it may more correctly be 

 said, that the sac of the yolk constitutes or is converted into 

 the intestine itself As development proceeds, the embryo be- 

 comes of an oval and then of a long shape. The spinal canal 

 is lengthened by the sprouting out of the tail from the serous 

 layer, and the intestine becomes more tubular in its form. I 

 have found the heart on the fifth day while the embryo is yet 

 enclosed in the egg, but I have not been able to follow the pro- 

 duction of the bloodvessels at the same time, owing to the dark 

 colour of the animal, and the transparency of the blood. 



The ova of the Aquatic Salamander, in which changes occur 

 very similar, in the early stages, to those in the frog, are much 

 more suitable to the observation of the phenomena of develop- 

 ment, as the germinal membrane is of a light colour, and the 

 coverings of the egg quite transparent. The excellent memoir 

 of Rusconi* has made known many facts relating to the de- 

 velopment of the larva of this animal. In the spring of the pre- 

 sent year, JJr Sharpey and I investigated the formation of its 

 rudimentary parts, and it appears, from our observations, that 

 the mode of development of the water newt resembles very 

 closely that of the frog. See Figs, of this. We have observed 

 the production of the heart and bloodvessels at a very early 

 period. Some days before the little foetus quits the egg, there 

 is seen on the abdomen an irregular sort of network, the bran- 

 ches of which, in a day or two, becoming more distinct, assume 

 a more regular disposition, and are collected into one trunk near 

 the heart (Fig. 9, A). At this time the heart, of a very simple 

 form, is distinctly seen contracting on the fore part of the neck , 

 but I have not yet succeeded in tracing this organ to its first 

 formation (/<). 



• Amours des Salaniandi-es Aqiiatiques, et developpement dii Tetard, &c. 

 INIilan, 1821 ; of which an account is given by Daniel Ellis, Esq., in Edin- 

 burgh Philosophical Journal, vol. ix. 1823. 



