EMBRYONIC DEVELOPMENT 



203 



In Ascaris megalocephala (page 200) the first distinguishing mark of 

 somatic cells is their early division. In the second cleavage of the 

 fertilized egg, one cell divides earlier than the other. Thus in Fig. 

 174A, B the left cell is ahead of the right in division, and it gives rise in 

 later cleavages only to somatic (sterile) cells. The cell which lags 

 behind gives rise to both somatic and germ cells. As the 4 cells derived 

 from this cleavage begin to divide to produce 8 cells, a second mark of 

 somatic cells becomes evident (Fig. 174C). The middle portion of each 

 of their chromosomes breaks up into many small pieces, which continue 



c D. 



Fig. 174. — Cleavage of the fertilized egg in Ascaris megalocephala, showing distinction 

 of somatic and germ cells. A, second cleavage, in which cell on left, in more advanced 

 stage of division, is somatic; B, later stage of second cleavage, with cells in same relative 

 positions and same relative states of advancement; C, third cleavage, with chromosomes of 

 three cells (somatic) fragmenting and losing their ends, those of the fourth remaining 

 intact; Z), fourth cleavage, with chromosomes fragmented in six cells (one hidden), becom- 

 ing fragmented in one (middle right), and remaining intact in one (upper right). All germ 

 cells are descended from the last-named cell. (Schematized from account by Fogg in Journal 

 of Morphology and Physiology.) 



as chromosomes, while the ends of the original long chromosomes are 

 thrown off into the surrounding protoplasm where they degenerate. 

 Three of the 4 cells lose chromatin in this way, and all these give rise 

 later only to somatic cells, while the one which retains its chromosomes 

 intact (upper right in C) produces both germ and somatic cells. In 

 each of the next two cleavages, in one of the cells that had retained 

 whole chromosomes, these chromosomes break up into small fragments 

 and lose their ends in the cytosome (D). Thus at the 32-cell stage 

 there is only 1 cell with long chromosomes like those of the fertilized egg. 

 In subsequent divisions of this cell there is no further loss of chromatin, 

 and all its descendants become germ cells. The other 31 cells have 

 fragmented chromosomes, and all their descendants are somatic cells. 



