90 OLIGOCHAETA 



The more prevalent type is seen in all earthworms except certain Eudrilidae, 

 and in a large number of the aquatic Oligochaeta; it is met with, for example, in 

 the Lumbriculidae and most Tubificidae. Yejdovsky has described in great detail 

 the facts for Rkynchelmis which I shall therefore take as an instance. The youngest 

 eggs are indistinguishable from the mass of ovarian cells among which they lie in 

 the egg-sacs ; the cells appear to be amoeboid in their youngest stages ; this is 

 inferred from their frequent pear-shaped character and from the fact that they are 

 not always in continuous contact. Any of these cells, it may be inferrred, may 

 develop into ova. As the egg-cell grows its peculiar characters already described 

 gradually differentiate themselves; the nucleolus is at first of course single and its 

 bipartition has been observed; no share whatever appears to be taken in the 

 development of the ovum by the surrounding cells ; at any rate no changes are 

 noted in them by Yejdovsky. In lAimbricus the mature ovum has, what it has not 

 in Rhynchelmis, a distinct follicular layer of flattened cells ; it is possible that these 

 cells do bear a part in the maturation of the ovum ; but apart from this follicle no 

 changes are observable in the remaining cells of the ovary which are not on the 

 way to become ova. 



The second method of egg-development differs from that just described in 

 the important fact that certain of the cells of the ovary do apparently take a 

 share in the formation of the ovum by contributing to its nutrition ; this way 

 of development has at present only been observed in Eudrilus by myself and 

 HoEST. Our observations agree in all essentials. The development here, as in 

 Rhynchelmis and many other worms, takes place in the egg-sacs. At first the 

 young ova are seen lying among a quantity of indifferent cells; any of these, it is 

 to be presumed, possess the capability of becoming ova ; later the cells in the 

 immediate neighbourhood of the more mature ova gradually break down; the out- 

 lines become obscured and the final stage reached is a mass of protoplasmic matter 

 in which neither cells nor nuclei can be any longer recognized, and which has a 

 fibrous appearance ; it is very possible, though I have no positive facts to go upon, 

 that the peculiar membrane already referred to as surrounding one pole of the 

 ovum is produced by this broken down mass of cells. The pores of the membrane 

 in question are figured by HoRST as penetrating the vitelline membrane, and he 

 thinks that they serve as the conduits of nourishment to the ovum. There is to 

 my mind an undoubted resemblance in the mode of development of the ovum in 

 Eudrilus to the formation of the Graafian follicle in the higher Mammalia; in the 

 latter, the liquor folliculi is produced by the breaking down of cells of the follicle, 

 at least partly; and this liquid may be fairly compared to the perhaps fluid 



