28 SEGMENTATION. 



appears relatively much earlier. It is formed when there are 

 eight segments marked out on the surface of tlie germinal 

 disc\ From Oellacher's^ account of the segmentation in the 

 fowP 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 seo-ments 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 seg- 

 ments of the disc are completely circumscribed, and only appear 

 to be so in surface views (vide PL i. fig. 6). Segmentation in 

 the Elasmobranch egg differs in the following particulars from 

 that in the Bird's egg : 



(1) The equivalent of the horizontal furrow of the Batra- 

 chian 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* and Klein^). 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^ figures, PL xxiii. fig. 19 — 21, peculiar 

 headings of the sides of the earlier formed farrows are dis- 

 tinctly shewn. No mention of these is made in the text, but 

 they are unquestionably similar to those I have described in 

 the Elasmobranch furrows. In the case of Elasmobranchs I 



^ Vide Elements of Embryology, p, 23. 



2 Strickefs Studien, 1869, Pt. i, PI. ir. fig. 4. 



3 Unfortunately Professor Oellacher gives no account of the surface appear- 

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

 uncertain to what period his sections belong. 



4 Zeitschriftflir Wiss. Zool. Bd. xxii. 1872. 



s Monthly Microscopical Journal, March, 1872. 

 ° Loc. cit. 



