642 FRANCIS M. BALDWIN 



nearly to the dorsal wall of the pericardium, and at the same time 

 it is reduced in its minor diameter so that it is a solid finger- 

 shaped spur of entodermal cells, reaching from its point of con- 

 tact with the pharynx toward the pericardial chamber. It is 

 shown in figure 20 pb. As yet it has no lumen, and its cells 

 show no marked histological differentiation from those of the 

 pharyngeal wall. 



In the 11 mm, larvae, although still connected with the pharyn- 

 geal wall, the axis of the anlage is changed noticeably, the distal 

 end now turning medially and caudally, so that in five successive 

 sections, only the first and second show the connecting stalk, the 

 remaining three pass through the distal portion of the anlage, 

 which now lies almost parallel to the long axis of the body. The 

 cells of the anlage have changed somewhat in their histological 

 appearance, especially in their distal region,' where they are much 

 darker in color (due probably to the loss of yolk) , and where the 

 tissue is less compact. A few cells in this region have separated 

 ' from the others and lie scattered in the connective tissue ; some 

 lying close to the medial wall of a small twig from the fourth 

 branchial artery, and others some distance from it. 



D. Amhlystoma larvae^ 13 to 15 mm. long. The postbranchial 

 anlage now has a position parallel to the long axis of the bod}^, 

 this change being due probably to the increase in size of the sur- 

 rounding parts. The connection with the pharynx is lost and the 

 body lies completely isolated — an elongate, irregular, illy defined 

 mass of cells in the connective tissue, between the medial aditus 

 laryngeus muscle and the more lateral cartilage anlage of the 

 fourth branchial arch, and the dorsal wall of the pericardium. A 

 few scattered cells in its anterior region persists as a remnant of 

 the connecting stalk; these extend upward toward the ventral 

 floor of the pharynx, while cells in its caudal region are scattered 

 in the connective tissue and are poorly defined. The central 

 region is solid and is nearly 50 micra long and about 10 micra in 

 average diameter, although in places it is very irregular as already 

 noted. This stage corresponds roughly to that described by 

 Maurer in Triton (p. 362), ''Schon vier T^age spater hat sich 

 dieser epithelia le Zellzapfen von der Schlundwand abgeschniirt 



