286 HISTORY OF THE HUMAN BODY 



glands specialized to produce this fluid may be termed salivary, 

 without reference to their position, and consist in mammals 

 of the parotid, sub-mandibular \_sub-maxillary BNA~\, sub- 

 lingual, and retro -lingual, the last associated with the sub- 

 mandibular and not found in all mammals. There also occur 

 other voluminous glands, with a structure similar to the 

 others, that secrete a clear fluid without salivary attribute. 

 Such are the molar glands of ungulates, or the voluminous 

 orbital gjand of the dog family (Canidae), which opens by a 

 duct into the mouth cavity in the region of the last upper 

 molar. 



Associated in origin with the pharyngeal region are cer- 

 tain organs of more or less uncertain significance and varied 

 destiny, the most prominent of which are the thymus and 

 thyre oid glands. These, although not connectecTTn function 

 with the dTgestive system, may be treated here because of 

 their origin. In the cyclostomes, which furnish the first, or 

 most generalized stage in this history, there develop seven 

 pharyngeal pockets on each side, each with a dorsal and a 

 ventral recess. About each of these there develops a prolifera- 

 tion of epithelial cells, forming an organ or organ-anlage. 

 These anlagen appear alike in structure, but in the higher 

 forms the differentiations from the dorsal series become col- 

 lectively known as the thymus, those from the ventral as epi- 

 thelial corpuscles. 



The later history of the thymus anlagen shows three ten- 

 dencies, (i) to become early separated from their layer of 

 origin, (2) to fuse upon each side to a single long organ, 

 often showing its segmented origin, and (3) to become re- 

 stricted in number, the more anterior ones being the first to 

 disappear. These are all seen in the accompanying diagram 

 (Fig. 79), which shows the phylogenetic steps in the process. 

 Thus, beginning with the seven anlagen of the cyclostomes, 

 the teleosts, which possess six pharyngeal pockets, show but 

 four, and these the most posterior. The first pocket has none, 

 that of the second is represented by a rudiment, which early 

 disappears. In the urodeles, in which five pockets appear, 



