XV 



NEMATODA 



443 



The nucleus of D undergoes reduction of chromatin, so that finally 

 only the cell P 4 is left in which the chromosomes retain their original 

 condition. This cell in the subsequent development divides only 

 once until the larval period is attained, and it is the parent cell of 

 the genital organs and gives rise to nothing else. 



It follows, therefore, that the tissues of the Nematode are con- 

 structed of cells which have lost a large part of the chromatin which 

 they had inherited from the egg cell, and which are, therefore, as 

 compared with the germ mother cell, degenerate cells. 



Zur Strassen has followed in detail the subsequent divisions of 

 the dorsal cell family, adopting for his purpose a most complicated 

 nomenclature, but nothing is gained by following him because there 

 is some evidence to show 

 that these cells are 

 equivalent to each other, 

 and that if, through 

 abnormal circumstances, 

 their number is increased, 

 it does not affect the 

 normal development of 

 the resulting embryo. 

 The daughters of A, as 

 we have seen, form a 

 median T -piece, and the 

 two sides of the beam of 

 this are occupied by the 

 daughters of 6 and /3- 

 respectively. 



Zur Strassen's object 

 was to discover if the 

 mutual displacements of 

 the cells of this family 

 could be accounted for 

 by the laws of surface- 

 tension, or whether there was evidence of cytotaxis, i.e. mutual 

 attractions and repulsions of cells based on their chemical qualities. 

 He found that surface-tension did undoubtedly cause rearrangement ; 

 it caused the cells to arrange themselves according to Plateau's 

 principle, i.e. in such a way that the total area of their boundary 

 surfaces was reduced to a minimum ; but he also found that it was 

 not the sole principle at work, because in some cases cells moved in 

 such a way as to directly contravene that principle, and hence there 

 was evidence that cytotaxis was at work (Fig. 347). 



Returning to the last stage minutely described, when both dorsal 

 family and ventral family consisted of eight cells, i.e. the 16- 

 cell stage, we may note that at this stage the egg forms an elliptical 

 blastula with a well-marked blastocoele. The first trace of the 

 invagination which will change the blastula into a gastrula is seen 



FIG. 347. Dorsal view of the segmenting egg of Ascaris 

 megalocephala in the 102-cell stage. (After Zur 

 Strassen. ) 



The daughters of A forming the permanent cross are light- 

 coloured. Those of b and (3 are cross-hatched. 



