58 EMBRYOLOGY. 



tion. It remains to be added to what has been previously said, that 

 this type is most frequent in the case of Invertebrates, and is to be 

 encountered among Vertebrates only in the cases of Arnphioxus and 

 Mammals. With the latter, however, there early appears a slight 

 difference in the size of the segments ; this has induced many 

 investigators to designate the cleavage of Amphioxus and Mammals 

 as unequal also. If I have not followed this suggestion, it is 

 because the differences are of a trivial nature, because the nucleus 

 in the egg-cell and also in its segments still occupies a central 

 position, and because the different methods of cleavage are in 

 general not sharply definable, but connected by transitional con- 

 ditions. 



Concerning Amphioxus, HATSCHEK states that at the eight-cell stage 

 four smaller and four larger cells are to be distinguished, and that 

 from that time forward in all the subsequent stages there is to be 

 observed a difference in size; and that the process of cleavage takes 

 place in a manner similar to that which will be subsequently 

 described for the Frog's egg. The egg of the Rabbit, concerning 

 which we have the painstaking investigations of VAN BENEDEN, 

 divides at the very outset into two segments of slightly different 

 size ; moreover, from the third stage of division onward there occurs 

 a difference in the rapidity with which the divisions follow each 

 other in the different segments. After the four cleavage-spheres 

 have been divided into eight, there is a stage with twelve spheres ; 

 this is followed by another with sixteen, and afterwards another with 

 twenty-four. 



I b - Unequal Cleavage. 



As a basis for the description of unequal cleavage we may employ 

 the Amphibian egg, the structure of which has already been con- 

 sidered. As soon as the egg of the Frog or Triton is deposited in 

 the water and is fertilised, and while the gelatinous envelope is 

 swelling up, its black pigmented hemisphere or animal half becomes 

 directed upward, because it contains more protoplasm and small 

 yolk-spherules, and is specifically lighter. The want of uniformity 

 in the distribution of the various components of the yolk also induces 

 an altered position of the segmentation-nucleus. Whereas the latter 

 assumes a central position in all cases in which the deutoplasm is 

 uniformly distributed, it invariably alters its location whenever 

 one half of the egg is richer in deutoplasm and the other richer in 

 protoplasm ; it then migrates into the more protoplasmic territory. 



