DIGESTIVE SYSTEM 



743 



oesophagus from which the respiratory system develops. It will be de- 

 scribed in more detail in our discussion of the respiratory system. 

 But, as there are certain more or less significant organs developed in the 

 pharyngeal region, it may be \vell to discuss them at this point. These 

 are especially the thymus and thyroid glands. It is customary to trace 

 the development of these two glands from the cyclostomes upward, be- 

 cause the cyclostomes furnish the first (more generalized) stage of de- 

 velopment of such glands, and thus make it possible to follow up con- 

 secutively any so-called advance from a lower developmental type to a 

 higher one. (Figs. 294, 434.) 



There are six pharyngeal pockets (except in the cyclostomes) de- 

 veloped on each side. Each of these pockets possesses a dorsal and a 

 ventral recess. It is around these recesses that a group of epithelial 

 cells develops an organ-anlage, all seemingly alike in the lower forms. 

 In the higher forms, however, the dorsal group soon forms the thymus 

 and the ventral forms what are called epithelial corpuscles. 



These thymus-anlagen may separate from the layer where they 

 originated or they may fuse into a single elongated organ, or they may 

 become constricted in number, the anterior ones disappearing. 



In the cyclostomes, there are seven anlagen. In the teleosts there 

 are six pharyngeal pockets, but only four anlagen, and these are all the 

 more posterior ones. 



tttyr 



Fig. 434. 



Thyroid and thymus glands with closely related organs. A, lizard; B, 

 Hen; C, Calf. car, carotid artery; h, heart; p.br.k., postbranchial bodies; 

 thym, thymus; thym' , point of thymus attachment; thyr, thyroid gland; tr, 

 trachea; v.jug., jugular vein. (After DeMeuron.) 



In the mammals it is the third pocket which produces the thymus- 

 anlage, although sometimes there is a tiny addition from the fourth. 



The epithelial corpuscles tend to disappear, but in amphibians they 

 become glandular and associate with the carotid artery to form carotid 

 glands. The number of these carotid glands varies in different groups of 

 animals. 



Immediately behind the last gill slit, in the floor of the pharynx, 

 there is a pair of evaginations. These have been termed supraperi- 



