BRANCHIAL DERIVATIVES PIED-BILLED GREBE 3] 



points the two bodies are in close contact and are in the process 

 of fusing, at others a distinct space separates them. The size 

 of the thymus IV at this stage, though somewhat variable, is 

 hardly more than a fourth that of the thymus III. In one em- 

 bryo the thymus IV has separated from its associated parathy- 

 reoid, in others the two are still narrowly connected, similarly to 

 the derivatives of the third pouch. At this stage the conditions 

 in the grebe, with respect to the derivatives of both third and 

 fourth pouches, correspond closely to those shown by Verdun 

 for his chick embryo of one hundred and eighty-eight hours. 



In embryos of nine days the thymus derivatives of the third 

 and fourth pouches are already fused. The double nature of the 

 resulting organ cannot in some cases be detected from its appear- 

 ance in the sections, but in others the two parts are as yet easily 

 discerned. The complete amalgamation of these derivatives is a 

 relatively rapid process and the thymus anlage of the fourth 

 pouch may, in the absence of the proper developmental stages, 

 be readily interpreted as of merely transitory nature. 



The parathyreoids III and IV at this stage lie side by side upon 

 the dorsolateral surface of the thyroid. The parathyreoid III is 

 medial in position and is considerably larger. The two are so 

 closely apposed that the division between them can be made 

 out only under high-power examination. In each of the bodies 

 of the right side traces of the original cavity still are found. In 

 the parathyreoid III the lumen is small, extending through 60 

 micra, but is sharply defined. It is situated near the lateral edge 

 of the body and is enclosed by a single layer of unthickened 

 epithelium, forming a vesicle partly surrounded by the modified, 

 cord-like masses of the gland. On the left side there are no 

 certain indications of the original cavities or of unmodified epi- 

 thelium in either of the parathyreoids. 



The postbranchial body in embryos of seven and one-half or 

 eight days may be partly or completely broken down into a 

 mass of loosely arranged cells, which is but poorly differentiated 

 from the surrounding mesenchyme. Because of this fact I came 

 to the conclusion in my earlier study that the body disappeared 

 soon after this period. Later stages at present available show 



