OF ARTS AND SCIENCES. 31 



We agree, substantially, with Klein ^"^ in the following statement : 

 " The fact that the parablast has, at the outset, been forming one 

 unit with what represents the archiblast (blastoderm, auct.), and, while 

 increasing, has spread, i. e. grown over, the yolk which underlies the 

 segmentation cavity, is, I think, the most absolute proof that the yolk 

 is as much different from the parablast as it is from the archiblast." 

 Rut we do not find in this a valid argument for the opinion that the 

 yolk of the teleostean ovum has no homologue in holoblastic vertebrate 

 Q^.j^_i8&i9 Q,^ fjjg contrary, we maintain, with Balfour,^ Waldeyer,^^ 

 and others, that the genesis of the ovum and the absence of any sharp 

 delimitation between the protoplasm and the deutoplasm before im- 

 pregnation, show conclusively that the latter is an integrant portion 

 of the ovum. The discovery by Balfour (No. 20, pp. 57, 90) and 

 Schultz" of a protoplasmic network (" Keimfortsiitze," Waldeyer) 

 extending throughout the yolk in the elasmobranch ovum, and the 

 discovery by Vau Bambeke "^ of " fines trainees protoplasmiques qui 

 partent en rayonnant de la base du disque et plongent dans la sphere 

 vitelline," in the unfecundated ovum of osseous fish, make it suffi- 

 ciently evident that the food-yolk cannot be regarded as adventitious 

 material in either case. "When we reflect that amon<r telolecithal 

 vertebrate ova a complete series of graduations in the segregation of 

 formative from nutritive material are found between the amphibian 

 ovum and the teleostean ovum, we find it impossible to accept any 

 theory of the constitution of the ovum which is not broad enough to 

 include both extremes. The positive evidence in fiivor of the view 

 here maintained does not lie, as supposed by Balfour, In the conver- 

 sion of the periblast ("parablast," auct.) into a cellular layer, but 

 in the actual cleavage of the yolk in some teleostean ova, as Jirst noted 

 by Mr, Agassiz. 



1" Klein, E. " Development of Common Trout." Quart. Jour. Mic. Sci., 

 XVI., p. 127, 187G. 



18 Ryder, John A. " Development of Silver Gar." Bull. U. S. Fish Com., I., 

 p. 295, 1881. 



li* "Observations on the Absorption of the Yolk, the Food, Feeding, and 

 Development of Embryo Fislies." Bull. U. S. Fish Com., II., p. 199, 1882. 



'° Balfour. Development of Elasmobranch Fishes, pp. 57, 89-90, 1878. 



21 Waldeyer. "Archiblast und Tarablast." Arch. f. Mik. Anat., XXII., 

 Heft 1, 1883. 



2- Schultz, Alex. " Zur Entwicklungsgcschichte des Selachiereies." Arch. 

 f. Mik. Anat., XL, 1875. 



23 Van Bambeke. "Recherches sur I'Embryologie des Poissons Osseux." 

 Mem. Cour. et Mem. de Sav. Etrang. de I'Acad. Roy. Belgique, XL , 1875. 



