Disputed Points in Teleostean Embryology. 207 



not a satisfactory one, for zona means a girdle or belt and no* 

 a hollow sphere like the membrane to which it is applied) 

 while radiata certainly does not mean " provided with minute 

 tubes vertical to the surfaces." The large memoir of Pro- 

 fessors M'Intosh and Prince (9) gives no fresh evidence as to 

 the development of the vitelline membrane ; the paper of 

 Iwakawa (12) to which they refer is not conclusive, nor are 

 Scharff's observations in his paper " On the Intra-Ovarian 

 Egg of some Osseous Fishes " (13). 



Segmentation. 



It will be convenient to take Balfour's summary of the 

 then state of knowledge in his ' Comparative Embryology ' 

 (1885) as our starting-point in considering this question. At 

 that time the process of segmentation had not been followed 

 step by step from its beginning to its end. Balfour says : — 

 " In hardened specimens a small cavity amongst the segmen- 

 tation-spheres may be present at any early stage ; but it is 

 probably an artificial product, and in any case has nothing to 

 do with the true segmentation-cavity, which does not appear 

 till near the close of segmentation. The peripheral layer of 

 granular matter continuous with the germinal disk does not 

 undergo division, but it becomes during the segmentation 

 specially thickened ; and, while remaining thicker in this 

 region, gradually grows inwards, so as to form a continuous 

 subblastodermic layer. In this layer nuclei appear which are 

 equivalent to those in the Elasmobranch ovum. A con- 

 siderable number of these nuclei often become visible simul- 

 taneously, and they are usually believed to arise spontaneously, 

 though this is still doubtful. Around these nuclei portions of 

 protoplasm are segmented off, and cells are thus formed 

 which enter the blastoderm and have nearly the same destina- 

 tion as the homologous cells of the Elasmobranch ovum. 

 During the later stages of segmentation one end of the 

 blastoderm becomes thickened and forms the embryonic 

 swelling, and a cavity appears between the blastoderm and 

 the yolk which is excentrically situated near the non- 

 embryonic part of the blastoderm. This cavity is the true 

 segmentation-cavity. 



" In Leuciscus rutilus Bambeke describes a cavity as 

 appearing in the middle of the blastoderm during the later 

 stages of segmentation. From his figures it might be 

 supposed that this cavity was equivalent to the segmentation- 

 cavity of Elasmobranchs in its earliest condition ; but 

 Bambeke states that it disappears and has no connexion with 



