154 



DEVELOPMENT OF THE FROG'S EGG [Cn. XIV 



cliial arches have greatly enlarged, and the efferent and afferent 

 vessels are now also united to each other in each arch by small 

 vessels (Fig. 47) or capillary tubes. The efferent vessels of 

 these two arches are also in communication with the dorsal 

 aorta of their respective sides. There is thus established at 

 this time a circulation of blood from the heart to the dorsal 

 aorta by way of the first and second branchial arches. 



In the third and fourth branchial arches the efferent vessels 

 have appeared. In the third arch the beginning of an affer- 



FIG. 47. AF 1 . Afferent branchial vessel. CV. Anterior cardinal vein. EF*. Effer- 

 ent branchial vein. G. Pneumogastric nerve. JV. Inferior jugular vein. L. 

 Capillary loop connecting afferent and efferent branchial vessels. N. Notochord. 

 O. Aorta. P. Pericardium. PH. Pharynx. SU. Suckers. V 4 . Fourth ven- 

 tricle. (After Marshall.) 



ent vessel is seen (Fig. 46). In the hyoid arch blood-vessels 

 appear, as we have seen, at an early stage of development and 

 seem to correspond to those in the branchial arches, but after 

 developing to a certain extent, they begin to degenerate. In 

 the mandibular arch no vessels have appeared at the time when 

 the larva leaves its capsule. Soon after this time a vessel de- 

 velops in this arch, and a small diverticulum arises from the 

 dorsal aorta (Fig. 46, B, MV), and later the two vessels unite. 

 The origin of the heart has been described, but as yet the 



