DEVELOPMENT OF ELASMOBRANCH FISHES. 27 



a similar occurrence takes place. Oellacher* and Gotte^ 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 in"dividually 

 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, w^hat for convenience may be called, 

 a vertical furrow which is followed by a second vertical furrow 

 at right ano-les 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 king- 

 dom, 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, 

 w^hich I have obseiwed, 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 

 by the irregular circumscription of a body of central smaller 

 spheres from a ring of peripheral larger ones (vide PL I. 

 figs. 3, 4 and 5). 



In the Bird the representative of the horizontal furrow 



1 Zeitschrift fiir Wiss. Zoolog'ie, Bel. xxiii. 1873. 

 a Archiv fiir Micr. Anat. Bd.^x. 1873. 



