TELEOSTEI. 69 



ovum very much reduced in size. It seems in fact very probable 

 that the Teleostei are in reality derived from a type of Fish 

 with a much larger ovum. The occurrence of a meroblastic 

 segmentation, in spite of the ovum being usually smaller-than 

 that of Amphibia and Acipenser, etc., in which the segment- 

 ation is complete, as well as the solid origin of many of the 

 organs, receives its most plausible explanation on this hypo- 

 thesis. 



The proportion of the germinal disc to the whole ovum 

 varies considerably. In very small eggs, such as those of the 

 Herring, the disc may form as much as a fifth of the whole. 



The segmentation, which is preceded by active movements 

 of the germinal disc, is meroblastic. There is nothing very 

 special to note with reference to its general features, but while in 

 large ova like those of the Salmon the first furrows only 

 penetrate for a certain depth through the germinal disc, in 

 small ova like those of the Herring they extend through the 

 whole thickness of the disc. During the segmentation a great 

 increase in the bulk of the blastoderm takes place. 



In hardened specimens a small cavity amongst the segment- 

 ation 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 granu- 

 lar matter, continuous with the germinal disc, does not undergo 

 division, but it becomes during the segmentation specially 

 thickened and then spreads itself under the edge of the blasto- 

 derm ; and, while remaining thicker in this region, gradually 

 grows inwards so as to form a continuous sub-blastodermic 

 layer. In this layer nuclei appear, which are equivalent to those 

 in the Elasmobranch ovum. A considerable number of these 

 nuclei often become visible simultaneously (van Beneden, No. 60) 

 and they are usually believed to arise spontaneously, though this 

 is still doubtful 1 . Around these nuclei portions of protoplasm 

 are segmented off, and cells are thus formed, which enter the 

 blastoderm, and have nearly the same destination as the homo- 

 logous cells of the Elasmobranch ovum. 



1 Vide Vol. II. p. 108. 



