378 HERBERT P. JOHNSON. 



not so frequent as one would expect if every egg requires them 

 for its proper development. It is possible there is some way of 

 getting rid of the nurse-cells after the ova have matured, possibly 

 by phagocytosis. This hypothesis is suggested by the fact that 

 the bunch of nurse-cells is most commonly located in a protected 

 place, usually in a sort of pocket between two closely-appressed 

 ova, or between an ovum and the intestine. The aspect of the 

 nurse-cells is closely that of young ova (Fig. 4). The principal 

 reason indeed for not regarding them as ova is the fact that in 

 any group there is never more than one cell that is unmistakably 

 an ovum ; it is always clearly distinguishable from the rest by 

 greater size and the presence of yolk granules. If they were all 

 ova one would expect a gradation from the largest to the smallest. 



What may be the unmodified 

 shape of the ovum, whether spher- 

 -A Q ical or oval, is impossible to de- 

 termine from eggs still confined in 

 the ccelomic spaces. In every in- 

 stance owing to pressure upon each 

 other or upon organs of the body 

 Fig. 4 .^Youn g ovum (o) with large the mature ova are more or less de- 

 nucleus, nucleolus, and a few small formed. Pressure of the alimentary 



yolk grains. The smaller cells are canal pro d UC es a Concavity (FigS. 

 considered to be nurse cells. X4-I2. x 



3 and 4) ; mutual pressure of two 

 or more ova in the same somite also produces a concavity in the 

 more yielding ovum, or else they are flattened against each other. 

 Flattening also occurs with the largest ova where they press 

 against the septa, extending as they do the entire distance from 

 septum to septum. This is sometimes the case even when one 

 or both of the adjacent somites contain no ovum, which gives 

 the impression of unyielding septa or else of very plastic ova. 

 The latter condition is no doubt existent. In one instance an 

 acicula and accompanying fascicle of setae, owing to contraction 

 of the retractor muscles have made a deep indentation in an ovum, 

 In other places ova are seen to be strongly constricted between 

 two expansions of the intestine ; in still other places ova are 

 wrapped, as it were, half-way around the intestine. 



Somites with two or three ova never occur near the ends of the 



