THE PROCESS OF CLEAVAGE. 61 



do they divide. At the fourth stage the 4 upper segments are first 

 divided by vertical furrows into 8, and then after an interval the 

 4 lower ones are divided in the same manner, so that the egg is now 

 composed of eight smaller and eight larger cells (fig. 32 16 ). After 

 a short resting stage the eight upper segments are again divided, this 

 time by a horizontal furrow, and somewhat later a similar furrow 

 divides the eight lower segments also (fig. 32 32 ). In the same 

 manner the 32 segments are divided into 64 (fig. 32 64 ). In the 

 stages which follow this, the divisions in the animal half of the egg 

 are still more accelerated relatively to those of the vegetative half. 

 While the 32 animal cells are divided into 128 segments by two 

 divisions which follow each other in quick succession, there are 

 still found in the lower half only 32 cells which are preparing 

 for cleavage. It thus comes to pass that, as the final result of the 

 process of cleavage, there exists a spheroidal mass of cells with entirely 

 dissimilar halves, an upper, animal half with small, pigmented 

 cells, and a vegetative half with larger, clear cells, containing more 

 abundant yolk. 



Erom the nature of the progress of unequal cleavage, as well as 

 from a series of other phenomena, one may lay down a general law, 

 first formulated by BALFOUR, that the rapidity of cleavage is pro- 

 portional to the concentration of protoplasm in the segment. Cells 

 which are rich in protoplasm divide more rapidly than those in which 

 protoplasm is more scanty and deutoplasm more abundant. 



As we have seen, the Frog's egg, by reason of the difference in 

 specific gravity between its animal and vegetative halves, by reason 

 of the heterogeneous pigmentation of its surface, by reason of the 

 unequal distribution of protoplasm and deutoplasm, and by reason of 

 the eccentric position of its nucleus, allows us to pass fixed and easily 

 determinable axes through its spherical body. On this account it is 

 an especially favourable object upon which to determine the question 

 whether the egg allows one to recognise in the position of its parts, 

 even before fertilisation, immediately after the same, and during the 

 process of cleavage, fixed relations to the organs of the fully developed 

 organism. This question has been tested by means of ingenious 

 experiments, especially by PFLUEGER and Roux, by the latter in his 

 " Beitrage zur Entwicklungsmechanik des Embryo." 



These have resulted in determining that the first cleavage plane of 

 the egg corresponds to the median plane of the embryo, so that it 

 separates the material of the right half of the body from that of the 

 left. Secondly, according to Roux, the position of the head- and tail- 



