PROCESS OF < I-KAVAGE. 01 



do they divide. At tin- fourth stair.- th- t up|. r ft gm Tits are first 

 divided hvvTtical furrows into 8, and then after an int-rval th>- 

 4 lower ones ar divid d in tin- same mann-T. so tliat the egg is 

 ..s.d of eight smaller and ( ight larger cells (tig. 32 16 ). A 

 a short resting stage the eight upp.-r segments are again divid. d. t his 

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

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

 manner tin- '.\'2 segnn nts are divided into 64 (fig. 32 64 ). In th 

 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 tli" .'>2 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 en? 



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

 o 11s, and a vegetative half with larger, clear cells, containing more 

 abundant yolk. 



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

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

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

 i . >i-t tonal to tlie concentration of protoplasm in the segment. ' 

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

 protoplasm is more scanty and deutoplasm more abundant. 



A- \ve have seen, the Frog's egg, by reason of the difference in 

 specific gravity between its animal and vegetative halves, by r- 

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

 une jual distribution of protoplasm and deutoplasm, and by reason or 

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

 determi nable axes through its spherical body. On this account it is 

 an especially favourable object upon which to determine the question 

 win-tiler the egg allows one to recognise in the position of its ; 

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

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

 organism. This (juestion has been tested by means of ingenious 

 exp riment>. especially by PFLUEOKR and Uorx. by the latter in his 

 "Beitrageiur EntwicktangBmeehanik das Ivnbryo." 



These ha\e resulted in determining that the tirst cleavage plane of 

 the egg corresponds to the median plane ..f the emhry<>. BO that it- 

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

 left. Secondly, according to Km x. th.- p -.he head- and tail- 



