SEGMENTATION. 239 



This very early appearance of a distinction in the germinal 

 disc between the extremity at which the embryo appears and 

 the non-embryonic part of the disc, besides its inherent interest, 

 has a further importance from the fact that in Osseous Fishes 

 a similar occurrence takes place. Oellacher 1 and Gotte 2 both 

 agree as to the very early period at which a thickening of one 

 extremity of the blastoderm in Osseous Fishes is formed, which 

 serves to indicate the position at which the embryo will appear. 

 There are many details of development in which Osseous Fish 

 and Elasmobranchs agree, which, although if taken individually 

 are without any great importance, yet serve to shew how long 

 even insignificant features in development may be retained. 



The segmentation of the Elasmobranch egg presents in most 

 of its features great regularity, and exhibits in its mode of 

 occurrence the closest resemblance to that in other meroblastic 

 vertebrate ova. 



There is, nevertheless, one point with reference to which a 

 slight irregularity may be observed. In almost all eggs seg- 

 mentation commences by, what for convenience may be called, 

 a vertical furrow which is followed by a second vertical furrow 

 at right angles to the first. The third furrow however is a 

 horizontal one, and cuts the other two at right angles. This 

 method of segmentation must be looked on as the normal one, 

 in almost all the important groups of the animal kingdom, both 

 for the so-called holoblastic and meroblastic eggs, and the 

 gradations intermediate between the two. The Frog amongst 

 vertebrates exhibits a most typical instance of this form of 

 segmentation. 



In Elasmobranchs the first two furrows are formed in a per- 

 fectly normal manner, but though I have not observed the 

 actual formation of the next furrow, yet from the later stages, 

 which I have observed, I conclude that it is parallel to one of 

 the first formed furrows ; and it is fairly certain that, not till a 

 considerably later period, is a furrow homologous with the hori- 

 zontal furrow of the Batrachian egg formed. This furrow 

 appears to be represented in the Elasmobranch segmentation 



1 Zeitschrift fur Wiss. Zoologit, Bd. xxm. 1873. 



2 Archiv fur Micr. Anat. Bd. ix. 1873. 



