THE OVUM. 33 



the cells which form the yolk-gland give rise to yolk particles 

 which enter the ovum, instead of to a layer of yolk-cells sur- 

 rounding the ovum. 



Amongst the marine dendrocoelous Turbellarians the ova are formed in 

 separate sacks widely distributed in the parenchyma of the body between 

 the alimentary diverticula. In these the ova undergo their complete develop- 

 ment, without the intervention of yolk-glands. 



The ovaries of the Nemertines more nearly resemble those of the 

 marine Dendrocoela than those of the Rhabdoccela. They consist of a series 

 of sacks situated on the two sides of the body between the prolongations 

 of the digestive canal. The eggs are developed in these sacks in a perfectly 

 normal manner, and in many cases become filled with yolk-spherules which 

 arise as differentiations of the protoplasm of the ovum. The protecting 

 membranes of the ova have not been accurately studied. In some cases 1 

 two membranes are present, an internal and an external. The former, 

 immediately investing the vitellus, is very delicate : the external one is 

 thicker and hyaline. 



The constitution of the female generative organs of the 

 Trematodes was first clearly ascertained by von Siebold (No. 12). 

 He originally, though not very confidently, propounded the 

 view that the germinal vesicles alone were formed in the ovary 

 and that the protoplasm of the ovum was supplied by the 

 yolk-gland. This view has long been abandoned, and von 

 Siebold (No. 13) himself was the first to recognize that true ova 

 with a protoplasmic body containing a germinal vesicle and 

 germinal spot were formed in the ovary. The Trematodes have 

 however not ceased to play an important part in forming the 

 current views upon the development of ova, and have quite 

 recently served Ed. van Beneden as his type in exposing his 

 general view upon this subject. 



His view consists fundamentally in regarding the secretion of the 

 yolk-glands, which in most cases merely invests the ovum, as homologous 

 with the yolk-spherules which fill the protoplasm of many eggs ; and he 

 considers the part of the ovary where in most forms the ova receive their 

 supply of yolk particles, as equivalent to the vitellarium of the Platy- 

 elminthes. He further appears to regard the primitive state as that 

 exemplified in Trematodes, Cestodes, etc., and holds that the ovarian 

 types characteristic of other forms are secondarily derived from this, by 

 the coalescence of the primitively distinct vitellarium with the ovary proper. 



1 Amphiporus lactiflorius and Nemertes gracilis. M c lntosh. Monograph on 

 British Nemertines. Ray Society. 



B. II. 3 



