DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 247 



Fig. 13), place the efferent vessels for the first time in direct con- 

 nection with the truncus arteriosus. 



According to Maurer's description, this is not the case in Bana 

 esculenta, for in this species the efferent or primary vessel is at first 

 the only vessel in the arch, and opens directly into the truncus 

 arteriosus. On the formation of the afferent or secondary vessel, 

 the efferent vessel loses its connection with the truncus, regaining it, 

 however, at a stage corresponding to that shown in Fig. 13, and in 

 a manner exactly corresponding to what we have described in Bana 

 tempwaria. This direct communication between afferent and efferent 

 vessels is of great physiological importance, for as soon as it is 

 established a path is open for the blood from the heart to the aorta 

 without passing through the gills ; and it is the gradual enlargement 

 of this direct passage that leads to the atrophy of the gills, and the 

 conversion of the animal from a gill-breathing to a purely air- 

 breathing form. 



The aperture or channel of communication between the afferent 

 and efferent vessels of the first branchial arch is of especial interest, 

 as it is in direct connection with it that that problematical structure, 

 the carotid gland, is formed at a later stage. 



The development of the carotid gland takes place in the following 

 manner: At 12 mm., as shown in Figs. 13 and 14, the lingual 

 artery and first branchial efferent vessel form one continuous vessel. 

 At the base of the lingual artery is a small bulb-like swelling, which 

 was present at an earlier stage, while the lingual artery was still an 

 independent vessel (Fig. 11). Immediately beyond this bulb the 

 afferent and efferent vessels are in direct communication, the passage 

 being a single narrow one, traversing a small plate of the deeply 

 staining cells described previously.* In the later stages this passage 

 becomes plexiform, there being now three or four openings into the 

 afferent vessel, and about the same number into the efferent, one or 

 more of the latter opening directly into the bulb-like swelling at the 

 base of the lingual artery. This plexiform communication becomes 

 the carotid gland, the history of its formation showing that it is 

 not to be regarded as a persistent or modified part of a gill, but that 



* According to Blaurer this epithelial plate is budded off from the epithelium 



of th'' first, branchial 'left. Fid. "Kchilddriise, Thymus und Kiemenreste dcr 

 Amphibien," " Morphologisches Jahrbuch," xiii., 1888, p. 321. 



