28 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. 



general tendency of variation by studying numerous eggs of a single 

 species, for example in the case of Eana 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 ditteivncrs oivur between the eggs of different species. 



KM.. lj. Variations in topographical relations of early segmentation furrows in the egg of 

 Rana 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 



KM.. 17. Vertical <ectiun through apical portion of egg of Jchthyopkis sA an advanced 

 of segnientation. (After I', and K. Sarasin.) 



pai-ts of tin- egg that during what are ordinarily called the 



lion be yolk remains completely nncleaved. It would in 



be concluded from an inspection of these stages alone that the 



- a merohiastie ,,ne. Examination of later -asn iilaii<>n stages 



however 'hat the yolk does eventually segment although 



lily, 

 I']."" tin- whole il leemfl 1 he the case that the I'mdele 



