30 CHARLES EUGENE JOHNSON 



The lateral mass, which is solid, is the thymus of the fourth 

 pouch (thymus IV) ; the medial body, consisting of cord-like 

 cellular masses and containing a small cavity near its postero- 

 lateral surface, is the parathyreoid .IV. Figure 3 is a trans- 

 verse section through the fourth pouch of another embryo of 

 seven days, taken slightly anterior to the middle of the pouch, 

 where the thymus and parathyreoid are well differentiated from 

 each other and are being separated. The structural difference is 

 apparent. The thymus portion in this specimen is slender and 

 cylindrical and somewhat smaller relatively to the parathyreoid 

 than that of the preceding embryo, but, like the latter, can be 

 traced as a ridge-like thickening in the dorso-lateral wall of the 

 pouch, throughout its length, presenting a duplication on a 

 smaller scale of the conditions existing in the third pouch. The 

 close analogy will be seen by comparing figure 4, which is a 

 section through the posterior, vesicular portion of the fourth 

 pouch, with figure 5, a section through the corresponding re- 

 gion of the third pouch in another embryo. The degree of 

 differentiation in the two pouches is nearly the same. 



The postbranchial body has lost its connection with the 

 pharynx and is now a closed vesicle, of fusiform shape, with 

 clear-cut lumen. Its position is still medial to the interspace of 

 the third and fourth aortic arches, at the level of the oesophagus. 



In embryos of eight and one-half days the thymus III is a nar- 

 row, greatly elongated body w^hose caudal third, approximately, 

 lies in the original position of the gland, just ventrad of the 

 jugular vein, while the anterior, greater portion, which is more 

 or less irregularly lobed, pursues a spiral course about the lat- 

 eral side of the vein and comes to lie along its dorsolateral sur- 

 face; the caudal portion is very slender and its extremity is nar- 

 rowly attached to the parathyroid III. The final separation of 

 the two bodies is apparently accomplished soon after the ninth 

 day, in the manner suggested by Helgesson for the sparrow, by a 

 process of atrophj^ and attenuation of the connecting portion of 

 the thymus. 



The thymus IV has correspondingly increased in length and 

 lies along the lateral side of the thymus III (fig. 6). At some 



