148 



O. C. GLASER. 



of the velum carries the external kidneys outward and still 

 farther away from the embryo, until finally they are far off at the 

 sides of the body and hang down from the under side of the 

 velum (Fig. 5). In this position the organs, which are white, 

 viscid masses of ovate shape, having at the height of functional 

 activity an average width of .7 mm., a length of i.o mm., and a 

 depth of .9 mm., remain throughout the life of the veliger until 

 this begins to assume the adult form. When that stage is reached 

 the external kidneys begin to decrease in size, to degenerate, and 

 the nuclei of the cells to disappear. Finally, in most cases, the 

 kidneys are dropped off before the velum is resorbed. There 

 are exceptions to this however, for often a much more intimate 



si^s^iPtei^s 



FIG. 6. Vertical section through external kidney showing the intimate relation 

 which it mav have with the velum (vel. }. Leitz J - 



2 



relation exists between the velum and the excretory organs hang- 

 ing down from its under surface. This greater intimacy is well 

 illustrated by Fig. 6, which is a typical section of these cases. 

 In this organ, after the original lumen had been filled by the plug 

 cells, the fully developed velum itself grew into the small pit 

 which remained at the proximal (final anterior) end of the 

 kidney. In this instance the velum was so crowded in the 

 remaining hollow of the external kidney that the velar cavity 

 itself was entirely obliterated by the cloj-e approximation of its 

 upper and lower walls. In cases of this kind the velum drops 

 off with the external kidney when the veliger assumes the adult 

 form. 



