THE OVUM. 27 



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 till 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. 



This appears to me a case of putting the cart before the horse. To my 

 mind the vitellarium is to be regarded, as has already been suggested by 

 Gegenbaur, Hallez, etc. as a special differentiation of the primitively simple 

 ovarian tube, and the instances of Macrostomum and Prorhyncus just cited 

 appear to me to indicate some of the steps in this differentiation. In 

 Macrostomum the cells of the lower part of the oviduct simply supply a 

 kind of nutriment to the ovum in the form of granular yolk particles, 

 while in Prorhyncus the yolk-cells of the lower part of the ovarian 

 tube form a complete investment of independent cells for the ovum. If 

 this lower part of the ovarian tube were to grow out as a special 

 diverticulum we should have produced a normal vitellarium. But even 

 with the above modification the theory of van Beneden appears to me not 

 completely satisfactory. The view that the yolk-spherules are of the same 

 nature as the yolk-cells is mainly supported by the case of Prostomum 

 caledonicum, where the vitellarium produces the yolk particles which fill 

 the ovum. The cases of Prorhyncus and Macrostomum give a different 

 complexion to that of Prostomum caledonicum. From the first of these 

 especially it appears that, even when normal yolk-cells surround the ovum, 

 yolk particles can be deposited independently in the protoplasm of the 

 ovum. 



The most probable view of the nature of the vitellarium is that 

 of Gegenbaur, Hallez, etc., according to which it is to be regarded as a 

 specially modified part of the ovarian tube. On this view the nature 

 and function of the yolk-cells admit of a fairly simple explanation. 

 They are to be regarded as primary germinal cells like those in the 

 ovaries of Hydra, Tubularia, etc., which do not become converted into 

 ova. Like these cells they may in some instances, Macrostomum, 

 Prostomum, etc., serve directly in the nutrition of the ovum. In 



