THE ANATOMY. REPRODUCTIVE SYSTEM 89 



yolked ovum, is spherical ; it has a peripheral and extremely fine membrane, beneath 

 which is a dense layer of protoplasm ; connected with this is a protoplasmic net- 

 work which ramifies through the entire egg and in the meshes of which are 

 contained the yolk-spherules. The spherical nucleus is surrounded by a distinct 

 doubly-contoured membrane. Around this membrane is a dense layer of protoplasm 

 with a radial arrangement of its particles ; but the membrane seems to be imper- 

 forate. The contents of the nucleus show an obscure meshwork in the interstices of 

 which is a granular nucleoplasm ; there are as a rule two nucleoli, of a spherical 

 contour ; the substance of these can be differentiated into two layers ; outside there 

 is a radially striate coat within which is the granular core. The ovum is elaborately 

 figured and described by VEJDOVSKY, to whose work reference must be made for 

 further details (9). 



Among the Megadrili the most aberrant ova are those of the Eudrilidae. Like 

 other Megadrili they are of small size owing to the small development of the 

 yolk. As in Lumbricus, there is a membrane covering the egg externally; but 

 this membrane is greatly developed in many Eudrilids. In Lumbricus it is 

 an excessively fine membrane as in Rhynchelmits ; but in Hyperiodrilus there 

 appear to be two distinct membranes. The ovum is surrounded by a very 

 thick, darkly staining membrane which is traversed by numerous pores ; beneath 

 this is a fine membrane which I regard as the probable equivalent of the vitelline 

 membrane of Lumbricus. The egg-protoplasm has a distinctly reticular arrangement, 

 and the nucleus has also a membrane separating it off from the surrounding 

 protoplasm. The ova of Heliodrilus appear to possess much the same structure. 

 In Eudrilus the ovum has a thick membrane, exactly like the thick outer membrane 

 of the ovum of Hyperiodrilus in minute structure; but it has the remarkable 

 peculiarity of being confined to one pole of the ovum ; it does not extend right 

 round. The structure of the ovum of this Annelid has been treated of by HORST 

 and myself. HORST (8) speaks of this peculiar membrane as a protoplasmic mass 

 formed of filaments which have the appearance of cilia. The membrane must, I 

 think, in spite of the fact that it is only partial, be compared to that of Heliodrilus. 

 The comparison seems to favour my opinion that the membrane in question is not 

 a product of the ovum, but is produced by the modification of the follicular cells 

 surrounding the growing egg. If it were a product of the egg protoplasm it would 

 surely surround the entire circumference of the egg in Eudrilus no less than in 

 Heliodrilus. I shall recur to this ' membrane ' in describing the egg-development. 



Development of the ova. There appear to be three types of egg-development in 

 the Oligochaeta. 



