6 EMBEYOLOGY OF THE LOWEK VERTEBRATES ch. 



the third furrow appears, not at the equator 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 this 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 be roughly 

 equivalent in amount upon the two sides of the 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 being 

 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 first 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 Figs. 

 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 or 

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

 wards towards the abapical pole. This phenomenon appears to be 

 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 further 

 progress tends to become irregular. New furrows make their 

 appearance — latitudinal, and vertical or meridional — and the surface 

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

 substance becomes cleaved or split apart into corresponding blasto- 

 meres as the superficial furrows gradually deepen into slits. 



At somewhere about this period there begins a new type of 

 mitotic division in which the individual blastomere becomes split 

 in a direction parallel to a plane tangential to its outer surface, so 

 that it divides into an outer blastomere visible in surface view and 

 an inner one concealed in the interior of the egg. 



With the further progress of segmentation the blastomeres 

 divide over and over again, so that eventually the egg is converted 

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

 with which the cells divide bears a rough inverse relation to the 

 richness of their contents in yolk. Dead inert yolk tends to cause 

 the cell to lag behind in the process of division, and the result of 

 this in telolecithal eggs is that the difference in size between 

 micromeres and macromeres becomes more and more marked as 

 segmentation goes on — the lower and more richly yolked segments 



