THE VITAL PHENOMENA OF THE CELL 



225 



touch each other. At the animal pole they meet in a short trans- 

 verse farrow, the animal cleavage line. The pointed ends of the 

 two remaining cells, which do not come in contact with the pole, 

 meet this line at its extremities. A similar arrangement is seen 

 at the vegetative pole : here the two cells, which did not touch 

 the animal pole, meet along a vegetative cleavage line, which is 

 always in such a position that if both lines were projected upon 

 a common plane they would intersect at right angles. Here 

 the four cells, which are obtained by quartering the original 

 cell, are not of the shape of ordinary quarters of a sphere. 

 Each has a blunt and a pointed end, the latter being directed 

 towards the pole of the egg. Each pair of 

 cells formed from a hemisphere are so ar- 

 ranged that similar ends point in opposite 

 directions. 



A corresponding arrangement of the first 

 four cleavage cells has been described by von 

 Rabl in the eggs of Plartorbis, and by von 

 Rauber (VI. 56) in Frogs' eggs. The latter 

 has entered into more details than the former. 



Similarly in oval eggs, in which, according 

 to our law, the first division plane is transverse 

 to the longitudinal axis, distinct separations 

 of the cells from each other occur during the 

 second cleavage, which is vertical to the first. 

 In consequence, well-marked cleavage lines 

 appear, as is seen in Fig. Ill in the egg of 

 Ascaris n ujrovenosd . 



Fig. 114. An egg of 

 Ascaris nigrovenosa with 



four segments. (After 

 Auerbach, PI. IV., Fig. 

 19.) 



lb. Unequal Segmentation. 



Unequal division comes naturally after equal. It is most- 

 generally caused by the unequal distribution of the protoplasm 

 and yolk substance in the cell. The Frog's egg, in which polar 

 differentiation has occurred, will serve as an example of this. 

 There, as has already been stated, the nucleus is situated in the 

 upper or animal half of the sphere (p. 217). Now when division 

 is about to occur, the axis can no longer lie in any one of the. 

 radii of the egg, for, in consequence of the unequal division of the 

 protoplasm in the egg space, it is influenced by that part of 

 the egg, which is pigmented and rich in protoplasm ; this portion 

 rests like a skull-cap upon the more transparent deutoplasm-con- 



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