PHAKYNGEAL DERIVATIVES OF AMBLYSTOMA 641 



able differentiation between cells of the pharynx and those of the 

 postbranchial anlage, both containing a considerable amount of 

 yolk. 



B. Ambly stoma larvae, 9.5 mm. long. In this stage there is 

 considerable advance in the development of the postbranchial 

 body. By proliferation, cells have been pushed in a ventral 

 direction from the thickened anlage, so that they now form a well 

 defined, short-stalked, cylindrical body, protruding vertically 

 downward toward the dorsal wall of the pericardium. This 

 anlage is about 40 micra long, and between 25 and 30 micra in 

 diameter. The cells of the anlage are closely packed; the body 

 is solid; and as yet no histological differentiation is apparent. 

 That this early anlage is solid in Amblystoma, and remains so for 

 a considerable length of time after the pharyngeal pouches have 

 opened to the exterior, would seem to be a point (though not 

 conclusive) in favor of Maurer's interpretation of it as a 'post- 

 branchial ' structure, and not a rudimentary sixth pouch, a view 

 which Greil ('05) opposes. On the other hand, this solid struc- 

 ture in Amblystoma is quite different from the early anlagen of 

 the bodies described by Miss Piatt in Necturus, where she finds 

 them — one on either side — forming ''small vesicles immediately 

 below the floor of the branchial chamber, with which they are still 



connected by stems They are formed by a single 



layer of yolk-laden cells whose nuclei are near the surface of the 

 vesicle, while the yolk granules are aggregated at the centre." 

 Figure 15 is a transverse section through the postbranchial 

 anlage {ph.) at this stage, and shows' its relation to the surrounding 

 structures. This section is just caudal to the posterior wall of the 

 ear, and cuts the glossopharyngeal and vagus ganglia, as well as 

 the internal jugular vein and the first and second efferent bran- 

 chial arteries. It also shows the lateral extensions of the suc- 

 cessive pharyngeal pouches (third, fourth and fifth visceral) 

 which extend to varying distances toward the ectoderm of the 

 lateral surface of the body, and between which occur the anlagen 

 of the branchial cartilages. 



C. Amblysto77ia larvae, 10 to 11 mm. long. In the 10 mm. larvae, 

 the anlage has lengthened (now about 60 micra long) and reaches 



