CHAP. XL.] 



CLEAVAGE OF THE YOLK. 



573 



two parts, in the centre of each of which a nucleated cell was situated. Each 

 of these cells again subdivides, and corresponding portions of the yolk sur- 

 round the cells resulting from the original sub-division. This process con- 

 tinues to be repeated until at last the individual cells cannot be distinguished, 

 and a granular mass results, from which the young worm is gradually evolved. 

 The division of the yolk appears to result from an attraction existing between 

 its particles and the cells, which divide previous to the subdivision of the 

 yolk-mass. 



In the cephalopoda, only a portion of the yolk undergoes subdivision, the 

 remainder serving for the nutrition of the embryo during incubation ; hence 

 the division into germ-yolk andfood-yolk. Amongst the vertebrata, this type 

 is followed in the class of fishes and birds, and in many reptiles. 



In the development of the ovum of ascaris dentata, Kolliker has found that 

 a totally different process occurs. In this entozoon, the first embryonic cell is 

 developed in the centre of the yolk after the disappearance of the germinal 

 vesicle, and grows at the expense of the yolk substance, instead of causing 



Fiff. 270. 



Multiplication of cells in the yolk. The first four figures, are ova of Ascaris dentata ; the 

 ' remainder, of Cucullanus elegans. After Kolliker. 



the latter to collect around it. This cell divides into two, which appropriate 

 nutriment in the same manner ; and further division and subdivision occurs, 

 until at last the yolk entirely disappears, and its place is occupied by a num- 

 ber of cells, resulting from the division of the primary cell, and the absorption 

 of the yolk-mass by the subsequent cells. 



Among the invertebrata, therefore, we have seen three distinct types upon 

 which the division and subdivision of the yolk occurs ; but in all we notice 

 the disappearance of the germinal vesicle, and the formation of a new " em- 

 bryonic" cell, by the division of which a large number of cells is at length 

 produced, giving the ovum the appearance of being composed of a granular 

 mass, from which the embryo is at length formed. 



In the mammalia, it is the first of these three modes which is followed. 

 The entire yolk divides, its particles collecting themselves round cells result- 

 ing from the division of the embryo-cell, until the yolk-membrane appears to 

 be entirely occupied by granular contents ; in this stage it is often termed 

 the " mulberry mass." 



