BRANCHIAL DERIVATIVES PIED-BILLED GREBE 29 



interspace of the third and fourth aortic arches instead of the 

 fourth and fifth. Its posterior terminal portion is in such close 

 apposition to the medially directed end of the third pouch, that 

 only by careful examination under high power of the microscope 

 can it be determined that the two structures are not actually 

 continuous. The relations of the postbranchial body to the third 

 pouch are thus in this stage almost identical with the earlier 

 relations of the body to the fourth pouch, and create an ap- 

 pearance of genetic connection with this pouch — an appearance 

 that becomes all the more convincing in the absence of proper in- 

 termediate stages. The forward movement of the postbranchial 

 body is evidently the result of traction exerted by the growing 

 pharyngeal tube. The right postbranchial body has been re- 

 duced to a slender strand of loosely associated cells, in some 

 parts of which, however, traces of the original lumen may still 

 be discerned. 



At seven days the process of separation between the thymus 

 and parathyreoid III has made considerable advance. The entire 

 pouch may, in some cases, have rotated to a certain degree on 

 its long axis, bringing the parathyreoid into a more nearly ventral 

 position. The thymus III (fig. 11) is an elongate compact body, 

 thicker and nearly cylindrical in its anterior portion, and flatter 

 posteriorly where it is united to the parathyreoid by its medial or 

 ventral surface. The parathyreoid contains a small lumen whose 

 immediate walls may in some cases consist of a single layer of 

 low cells, overgrown by adjacent rapidly proliferating areas; in 

 others the proliferating process has extended so that the lumen 

 appears merely as a slit in the mass. 



The fourth pouch presents a form very similar to the third, 

 but is much smaller, being but shghtly longer than the parathy- 

 reoid portion alone of the latter pouch; this great difference in 

 relative size has resulted from the rapid growth of the thymus 

 portion of the third. The entire pouch is shown in medial 

 surface view in figure 12, from a model made to the same scale 

 as that illustrated by figure 11. To be noted is the division of 

 the pouch into two main masses which have the same relation to 

 each other as the thymus and parathyreoid of the third pouch. 



