202 THE EVOLUTION OF MAN. 



or black-coloured egg is minutely examined, the uppei 

 half appears darker than the lower. In some kinds, the 

 centre of the upper half is blacker, wliile the corresponding 

 centre of the lower half is of a whiter colour.^'' This marks 

 a distinct axis of the egg with two different poles. In order 

 to give a clear conception of the cleavage of this egg, it is 

 best to compare it to a globe, on the surface of which 

 different meridian and parallel circles are marked. For 

 the superficial boundary lines between the different cells, 

 which result from repeated division of the egg-cell, have 

 the appearance of deep furrows on the surface, for which 

 reason the whole process has received the name of "the 

 furrowing" (i.e. cleavage).^^ But this so-called cleavage, 

 which was formerly regarded with astonishment as a very 

 wonderful process, is, in reality, only an ordinary and often- 

 repeated division of the cells. Therefore the " cleavage- 

 globules," which result from it, are really true cells. 



Unequal cleavage, as we see it in the amphibian egg, is 

 especially marked by the fact that it begins at the 

 upper, darker pole — the north pole of the globe, according to 

 our simile — and proceeds slowly downwards towards the 

 lower, lighter pole, the south pole. During the egg-cleav- 

 age the upper, darker hemisphere is in advance, and its 

 cells divide more vigorously and quickly ; the cells of the 

 lower hemisphere, therefore, appear larger and less numer- 

 ous.^*^ The cleavage of the parent-cell (Fig. 31, A) begin? 

 with the formation of an entire meridian-furrow, which 

 starts at the north pole and ends at the south pole (B). 

 An iiouj- later, a second meridian-furrow arises in the same 

 way, and cuts the first at right angles (Fig. 31, C). The 

 S]>bere of the egg is thus divided into four similar segments. 



