28 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



general tendency of variation by studying numerous eggs of a single 

 species, for example in the case of Rana palustris Jordan and 

 Eycleshymer (1894) found amongst other variations in the mode of 

 appearance of the first furrows, those illustrated in Fig. 6 (p._ 12). 

 And similar differences occur between the eggs of different species. 



Fig. 16. — Variations in topographical relations of early segmentation furrows in the egg of 

 Rwna temporaria. (A, B after Morgan, 1897 ; C after Jenkinson, 1913.) 



The figure in each case represents a view of the apical pole of the egg. 



As regards difference in position of the furrows two of the 

 commonest variations are the following. At the four-blastomere 

 stage two blastomeres may be pressed outwards from the apical pole 

 as in Fig. 16, A. Again meridional furrows may be replaced by 

 vertical furrows as in Fig. 16, B and C. 



As regards' variations in time these are chiefly associated with the 

 retarding of segmentation in the lower yolk-laden segments. This 

 reaches its maximum, so far as Amphibians are concerned, in the 

 Gymnophiona, where segmentation spreads so slowly into the lower 





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Fig. 17. — Vertical section through apical portion of egg of Jchthyophis at an advanced stage 

 of segmentation. (After P. and F. Sarasin.) 



parts of the egg that during what are ordinarily called the segmenta- 

 tion stages the yolk remains completely uncleaved. It would in 

 fact be concluded from an inspection of these stages alone that the 

 egg is a meroblastic one. Examination of later (gastrulation) stages 

 however shows that the yolk does eventually segment although 

 tardily. 



Upon the whole it seems to be the case that the Urodele egg 



