m the Fcetus of Vertehratea Animals, 108 



able. The fourth arch communicates with the third arch, and 

 sends a returning vessel contributing to the formation of the 

 aorta ; the principal part of the trunk (p), however, descends 

 on each side of the vertebral column to the sac of the lungs, 

 upon the cells or compartments of which it is ramified with great 

 minuteness. 



Frog. — During the first part of foetal development, the re- 

 spiration of the larva of the common Frog is carried on by 

 nearly the same means as that of the aquatic salamander. The 

 rudimentary intestine, or part corresponding to the yolk sac, 

 seems to be covered by a network of venous vessels, to which 

 the liver holds the same relative position as in the salaman- 

 der. Branchial appendages, analogous in their structure and 

 relations to those of the salamander, are formed, and the tail, 

 as well as the general integuments of the body, appears to assist 

 the more special respiratory organs in changing the blood. 

 The external branchial appendages exist for a much shorter 

 time in the frog than in the salamander : they never become 

 highly developed, or capable of exposing a large quantity of 

 blood to the influence of the water ; but their place is supplied 

 at an early period by internal gills, corresponding in some re- 

 spects with those of osseous fishes (Figs. 13, and 14, H I). 



According to the observations of Baer *, before the external 

 gills become developed, the distribution of vessels in the neck 

 of the tadpole of the frog resembles in its simplicity that in the 

 fcetal fish and salamander. The aorta, rising from the bulb 

 and advancing forwards to the region of the pharynx, is divided 

 into two branches, one of which proceeds to each side oi the 

 neck, and gives successively in its course four branchial vascular 

 arches, which, winding round the branchial hoops, reunite with 

 one another on the upper side of the intestine to form the de- 

 scending aorta. 



The external gills generally attain their full size about the 18th 

 day of development (Fig. IS, H.) Soon after this period, they 

 appear to shrink and lessen : their free motion is now impeded by 

 the growth of an opercular fold of the integuments anterior to 

 them (Fig. 13. a), which, increasing more and more from before 

 backwards, gradually forms a cover which encloses them entire- 



• In his Geschichte des Froschembryo, in Burdach's Physiol. B. il. S. 222. 



