252 THE DUCTLESS GLANDS 



epithelial bud occurs in the region of the second gill-cleft, 

 which at first resembles the parathyroids from clefts 3 and 4, 

 but soon becomes distinguished by its relation to the branchial 

 arteries. Zimmerman states that there are no epithelial 

 elements, but only a proliferation of the vascular wall. Maurer 

 believes that the carotid body of Rana is homologous with the 

 carotid body of higher vertebrates. The matter is doubtful 

 in reptiles and birds, though Maurer believes that in Echidna 

 the origin of the structure is the same as in the frog. Schaper, 

 however, believes that in mammals the body is a development 

 of the wall of the artery. Maurer acknowledged that our 

 knowledge of the development is still very imperfect, and 

 suggests that under a single name several different structures 

 have been described. Kohn believes that the carotid body is 

 derived from the embryonic ganglion cells of the sympathetic 

 plexus. 



Microscopical Structure 



According to Kohn, there are four principal types of the 

 organ according to the arrangements of the cellular elements. 

 In the first type the body is compact and circumscribed. The 

 connective tissue is so finely divided that the cellular character 

 of the tissue is predominant. The carotid gland of the cat is a 

 good example of this type. (See Fig. 56.) 



In other cases the connective tissue is distributed into the 

 gland in larger amount, and so two new types arise. In one of 

 these the organ has a kidney-like form. At the hilus there is a 

 considerable accumulation of connective tissue with blood- 

 vessels and nerves. From this region radial septa run into 

 the interior, which divide up the organ into lobules after the 

 manner of a secreting gland. The lobular formation is found 

 typically in the carotid gland of the monkey (Macacus rhesus). 

 In man the gland is divided by its connective tissue into small 

 separate islets. 



The fourth type is exemplified in the carotid body of the 

 rabbit, which consists of scattered islets and strands of cellular 

 tissue. This may be called the diffuse type (Kohn). 



The cells are of variable, for the most part of considerable, 

 size. Their form is manifold ; they may be prismatic or 

 cylindrical, but they are frequently flat. Kohn does not deny 

 that there is a certain resemblance to an epithelial structui 



