617 



markedly flattened, the morphological relations clearly resembling 

 closely those in the 7.0 mm. embryo described in detail by Elze '07. 

 The ventral pocket of the fourth pouch is present as a conical ele- 

 vation from the cephalo-ventral aspect of the caudal pharyngeal 

 complex. Its axis is directed toward the region of the thyroid as in 

 the 5 mm. specimen. The more markedly flattened form of the ulti- 

 mobranchial at this stage, the direction of its axis and possibly its 

 relatively large size are clearly an expression of the growth relations 

 of the region, the marked neck bend at this period having an effect 

 on the form. The direction of the outpocketing is, however, roughly 

 parallel to the trachea and esophagus. 



The flattened form becomes speedily lost. In the 8.3 mm specimen 

 it is roughly circular in section and relatively short ; in the 9.4 mm. 

 embryo but slightly flattened, and in the larger embryos nearly cir- 

 cular in transaction (figure 2) and subconical in form. 



The extension from the complex toward the thyroid, the so-called 

 ventral pocket of the fourth pouch becomes lost during the same 

 period. Still well marked in the 7.5 mm. it is small at 9.4 mm., 

 slight in the 10 mm. embryo and not recognizable subsequently to 

 that stage. The absorption of the ventral pocket into the general 

 complex is also clearly a result of the change in tensions and shiftings 

 during the unequal growth of the region. Whether or not the ventral 

 pocket may be designated as Thymus IV as Tandler has done, seems 

 to the writer questionable. Statements, such as that of Schaefer '12, 

 that a thymus developes regularly from the fourth pouch, are clearly 

 erroneous. The occasional development of thymus tissue from some 

 portion of the caudal pharyngeal complex, together with the com- 

 parability of the ventral pocket, in position and the general character 

 of its epithelium, to the ventral pocket of the third pouch which 

 clearly undergoes thymic transformation, are responsible for the desig- 

 nation Thymus IV even though it has not been shown that such 

 Thymus IV bodies, when they occur, have such an origin. The 

 designation carries with it, however, the interpretation of instrinsi- 

 cally specific thymus-forming cells, and in as much as the factors 

 that determine thymus formation are largely unknown, such a specific 

 name may well be omitted. My conclusions, therefore, differ from 

 those of Tandler, who finds that pouch IV in man regularly pos- 

 sesses the anläge of a thymus IV, — more in theoretical interpretation 

 than in fact, although it would appear that the distinctness of the 



