ti EMBEYOLOGY OF THE LOWEli VERTEBRATES n. 



the third t'urr<i\v appears, not ;il the eipiator but at a level nearer the 

 apical pole, and is termed a latitudinal furrow, corresponding as it 

 does with a parallel of latitude upon a terrestrial globe. The distance 

 of tliis furrow from the equator, its degree of latitude so to speak, is 

 roughly proportional to the degree of telolecithality of the particular 

 egg, suggesting that the volume of living protoplasm may lie muchly 

 equivalent in amount upon the two sides of tin- division plane to 

 which this furrow gives rise. . 



When this third division is completed the egg consists of eight 

 blastomeres, the four on the apical side of the division plane lie ing 

 smaller (micromeres) than those on the abapical side (macromeres). 



The next furrows to appear are two in number and in the 

 simplest condition they are meridional, bisecting the angles between 

 the two tirst furrows. More frequently however those furrows 

 instead of traversing the pole of the egg are discontinuous at this 

 point and each is displaced somewhat so as to join the first or second 

 meridional furrow at a less or greater distance away from the pole. 

 To such a furrow we apply the term vertical (Fig. 2, v.,cf. also l-'i-s. 

 14 and 16, C). 



It is as a rule noticeable that meridional or vertical furrows tend 

 to become apparent first in their portions nearest to the upper <>i 

 apical pole of the egg, their lower ends gradually extending down- 

 wards towards the abapical pole. This phenomenon appears to In- 

 due to the retarding influence of the dead and inert yolk. The 

 proportion of this to the living protoplasm becomes greater and 

 greater as the distance from the apical pole is greater, and in 

 correlation with this the retarding effect becomes more and more 

 pronounced. 



After segmentation has reached the stage indicated its tnrther 

 progress tends to become irregular. New furrows make their 

 appearance latitudinal, and vertical or meridional and tlxe surface 

 of the egg takes on the appearance of a mosaic- work, while its 

 substance becomes cleaved or split apart into corresponding blast o- 

 meres as the superficial furrows gradually deepen into slits. 



At somewhere about this period there begins a new type of 

 initoiic division in which the individual hlastmneiv becomes split 

 in .1 din-ciioii parallel to a plane tangential to its outer surfa< 

 that it divides into an outer hlastomeiv visible in surface \ie\\ and 

 an inner one concealed in the interior of the egg. 



With tin- further progress of segmentation the blastnm- 

 divide over and over aga in, so that e\eiil uallv the egg is eonverled 

 into a very large number of small cellular elements. The rapidity 

 wilh which tin- e^lls divide bears a rough Inverse relation t" the 

 richness of 1 heir conlenl s in yolk. head inert \olk lends to cause 

 the '-ell to lag behind in the prooeSB of division, and the resiill of 



tllH in tclnlecit h;i| eggS is I hat the ditfeiVlice ill si/e between 



micronjei-es and macro meres becomes more and more marked as 

 segment at ion 'joes on ih,- |..\\er and mmv richly ynlked segments 



