240 DEVELOPMENT OF ELASMOBKANCH FISHES. 



by the irregular circumscription of a body of central smaller 

 spheres from a ring of peripheral larger ones (vide PI. 6, figs. 

 3, 4 and 5). 



In the Bird the representative of the horizontal furrow 

 appears relatively much earlier. It is formed when there are 

 eight segments marked out on the surface of the germinal disc 1 . 

 From Oellacher's 2 account of the segmentation in the fowl 3 it 

 seems certain, as might be anticipated, that this furrow is nearly 

 parallel to the surface of the disc, so that it cuts the earlier 

 formed vertical furrows and causes the segments of the germinal 

 disc to be completely circumscribed below as well as at the 

 surface. In the Elasmobranch egg this is not the case ; so that, 

 even after the smaller central segments have become separated 

 from the outer ring of larger ones, none of the segments of the 

 disc are completely circumscribed, and only appear to be so in 

 surface views (vide PI. 6, fig. 6). Segmentation in the Elasmo- 

 branch egg differs in the following particulars from that in the 

 Bird's egg : 



(1) The equivalent of the horizontal furrow of the Batrachian 

 egg appears much later than in the Bird. 



(2) When it has appeared it travels inwards much more 

 slowly. 



As a result of these differences, the segments of the germinal 

 disc of the Birds' eggs are much earlier circumscribed on all 

 sides than those of the Elasmobranch egg. 



As might be expected, the segmentation of the Elasmobranch 

 egg resembles in many points that of Osseous Fishes (vide 

 Oellacher 4 and Klein 8 ). It may be noticed, that with Osseous 

 as with Elasmobranch Fishes, the furrow corresponding with the 

 horizontal furrow of the Amphibian's egg does not appear at 

 as early a period as is normal. The third furrow of an Osseous 

 Fish egg is parallel to one of the first formed pair. 



In Oellacher's 6 figures, PI. 23, figs. 19 21, peculiar headings 



1 Vide Elements of Embryology, p. 23. 



2 Strieker's Studien, 1869, Pt. i, PI. II. fig. 4. 



3 Unfortunately Professor Oellacher gives no account of the surface appearance of 

 the germinal discs of which he describes the sections. It is therefore uncertain to 

 what period his sections belong. 



4 Z-eitschrift fur Wiss. Zool. Bd. xxn. -1871. 



5 Mo nthly Microscopical Journal, .March, 1872. K Loc. at. 



