DEVELOPMENT OF SUPRAPERICARDIAL BODY 393 



ability the power of growth is greatest in the elements which 

 represent the connecting-stalk. In this region, in any embryo, 

 from 36 to 230 mm. long, the process of budding or new tubule 

 formation can readily be studied. 



The formation of new tubules and the branching of tubules 

 will be discussed together, as both processes, so far as can be 

 determined, are analogous, up to a certain stage in their develop- 

 ment. At the site of a future tubule or branch a local prolifer- 

 ation of cells occurs. This is followed by a direct extension of 

 the lumen of the parent tubule into this bud. The process 

 may end here, resulting in simple branching, or the new sprout 

 may become constricted at its base and later become cut off, 

 forming a new isolated vesicle. The first step in the process — 

 a thickening in the epithelium of the parent tubule — is very 

 transitory, being followed almost immediately by an extension 

 of the lumen into the new cell mass. Usually the newly formed 

 cells are not numerous and form only a very small bud which 

 remains surrounded by the membrana propria (fig. 11). Their 

 nuclei are broadly oval or round and are closely crowded together. 

 The cytoplasm of the cells is small in amount and stains deeply. 

 Cell membranes could not be distinguished. These small buds 

 of cells are quite numerous and are found both on the sides and 

 at the extremities of the tubules. 



The extension of the lumen into the cell mass is probably due 

 to a more rapid growth of the wall of the tubule at this point. 

 When once a definite outpouching is established, the cells become 

 radially arranged around the cavity (fig. 12). The attachment 

 of the outpouching to the wall of the parent tubule shows a 

 slight constriction into which the surrounding mesenchymal cells 

 push. The epithelial cells forming the attachment are narrow 

 and elongated and closely crowded together on one side, due 

 to a slight twisting of the outpouching toward that side. A 



Fig. 14b Same section as 14a, but taken at a lower focus in which the tubules 

 are separate, showing the changes in the form and arrangement of the nuclei. 

 X 450. 



Fig. 15 Another specimen of the last stage of tubule formation showing 

 more clearly the rotation of the nuclei as the tubules separate; from an embryo 

 47.3 mm. long (S. C. 11); alum hematoxylin. X 450. 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 2 



