34 ECHINODERMATA. 



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 other cases they retain 

 their independence and serve for the late nutrition of the embryo. 

 In both instances they retain the faculty, normally possessed by 

 ova, of forming yolk particles in their protoplasm. 



ECHINODERMATA. 



(16) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." Nieder- 

 ::tch. Archivf. Zoologie, Vol. I. 1871. 



(16) C. K. Hoffmann. " Zur Anatomic d. Asteriden." Niederlandisch. Archiv 

 /. Zoologie, Vol. II. 1873. 



(17) H. Ludwig. "Beitrage zur Anat. d. Crinoiden." Zeil. f. -wiss. Zool., 

 Vol. xxvin. 1877. 



(1*) Job. Mliller. " Ucbcr d. Canal in d. Eiern d. Holothurien." Mailer's 

 ArfAiv, 1854. 



