454 



HISTOLOGY 



appears to be the case in some hydroids. This method of ingesting food- 

 laden cells is carried up into some higher forms as the tunicates, where 

 the growing egg cell ingests whole rows of surrounding yolk cells whose 

 nuclei persist for a while in its cytoplasm (Fig. 416). As a rule, where 

 the cells are thus eaten bodily they are neighboring reproductive cells 

 which are thus arrested in their career. This appropriation may even 

 go on after the eggs are laid and development has commenced. 



The more general method of yolk accumulation, however, is for the 

 growing ovum to join itself in close bodily contact with some cell that, 



FIG. 417. Five stages in the growth development of an oogonium of Myzostoma. The two 

 nurse cells (w.c.) increase in size as the ovum grows and finally fuse with its cytoplasm. (After 

 WHEELER.) 



if related by descent, comes from a more remote common ancestor and 

 is differentiated to act as a follicle cell or nurse cell. In this case the 

 follicle cell or nurse cell takes food from the blood and, after elaborating 

 it in some unknown manner, feeds it into the egg cell until there is a suf- 

 ficient supply. It usually ends by giving up its own substance until but 

 a dead remnant appears lying on the surface of the young ovum. 



In the annelid Ophryotrocha, Korschelt has shown that one single 

 nurse cell does all the work, attaching itself to the very small ovum and 

 feeding it both with food secured from the surrounding body cavity 

 fluid and with the contents of its own body until it appears as a mere 

 excrescence on the body of the full-grown ovum, now many times the 

 bulk of the two together when they started. 



