DEVELOPMENT OF THE BLOOD VESSELS IN THE PROG. 251 



several aortic arches. In front of the pericardial cavity the truncus 

 arteriosus divides at once into right and left branches, each of which 

 is still single at its base. Each branch, after a short course outwards, 

 divides into a smaller anterior branch, which is the afferent vessel for 

 the first branchial arch, and a larger posterior part which again 

 divides almost at once into the second branchial afferent vessel, and 

 a posterior vessel which after a short course outwards and backwards 

 divides into the afferent vessels for the third and fourth branchial 

 arches. 



In sections of tadpoles of about this stage the blood vessels are 

 partially filled with blood clots : these are not seen during the 

 earlier stages of development, in which the vessels appear empty 

 but for the corpuscles. These latter have by this time the shape 

 and appearance of those of the adult. 



4. The Blood Vessels of the Visceral Arches. 



a. The Mandibular Arch. — The pharyngeal artery, which is now 

 the only vessel referable to this arch, has the same relations as 

 before. It arises from the aorta rather further forwards, just before 

 the latter enters the skull as the internal carotid artery. 



b. The Eyoidean Arch has, as described in the last stage, no vessels 

 referable to the branchial series. 



c. The First Branchial Arch. — The afferent branch of the truncus 

 arteriosus runs outwards to the base of the first gill : here it enlarges 

 rather suddenly, and gives off branches to the filters on the first 

 branchial arch. It then continues along the arch, giving off numerous 

 branches to the internal gills. 



The efferent vessel communicates with the afferent at the base of 

 the arch by a carotid gland, which consists of two or three narrow 

 irregular channels of a spongy appearance running in the septum 

 between the two vessels, and opening into both of them. Beyond the 

 carotid gland the lingual artery runs forwards in the floor of the 

 mouth. In the branchial arch itself the relations of the efferent 

 vessel are the same as at the previous stage. 



d. a/nd e. The Second and Third Branchial Arches. — The vessels of 

 those arches have undergone scarcely any change as compared with 

 the 12 mm. stage. In both arches we have found the afferent and 

 efferent vessels to he in direct communication at the base of the gills : 



