DEVELOPMENT OF SUPRAPERICARDTAL BODY 395 



in the shape of the nucleus, could not be determined with cer- 

 tainty. Figure 146, is a view of the same two tubules shown 

 in figure 14a, but is taken at a lower focus in which the tubules 

 are separate. The nuclei of the adjacent, newly separated walls, 

 especially on the right side, are oval or triangular, giving an 

 appearance which suggests that the change in the axes of the 

 nuclei is due to a change in their shape. In figure 15, however, 

 in which the tubules are nearly completely separated, the nuclei 

 are elongated or broadly oval and show varying degrees of 

 obliquity which one would expect to find if the nuclei rotated 

 during the process of separation. As the majority of the branch- 

 ing and budding tubules show changes very similar to these 

 described for figure 15, it would appear that this process was 

 most probable and that the variations in shape of the nuclei 

 seen in figure 146, are due to the pressure of the adjacent tubule. 

 The tubules at first are separated only by a membrana propria, 

 but later the mesenchyma and blood vessels intervene. 



The cells in the wall of the fused tubules are too numerous 

 and the cell walls too indistinct to follow the change in position 

 of the cell itself, but in all probability the rotation of the axis 

 Of the nucleus represents a rotation of the entire cell. If this 

 be true this process would be comparable to a reversal of the 

 changes which have been described by Scammon ('15) in the 

 anastomoses of hepatic tubules in selachians. 



Figure 10 is from a section through the cranial portion of the 

 gland modelled in figure 24, showing the termination of the 

 elements B, C and D in small tubules. The dorsa'l tubule, A, 

 is a direct continuation of the epithelium of the pharynx. Two 

 small solid buds arise from the right border of the ventral vesicle. 

 Caudally the large vesicle tapers into a tubule which has become 

 enclosed by the developing fibers of the coracobranchial muscle. 



The epithelial cells of the tubules vary in shape, from low 

 columnar in the smaller to a high columnar in the larger tubules 

 and vesicle. The nuclei are either broadly oval or elongated 

 and occupy the basal portion of the cell. The pharyngeal epi- 

 thelium at this stage (47.3 mm.) is definitely stratified, showing 

 two or three rows of oval nuclei. Numerous mitotic figures 



