86 DIFFERENTIATION III 



Frog) but certainly does not in other cases, where it appears 

 that the old axis persists unaffected by the operation, not 

 marked by any visible differentiation of materials, but causally 

 related nevertheless to the symmetry of cleavage and develop- 

 ment. That this axis has not been affected by the centrifuge 

 hitherto does not, however, justify the assumption that it 

 cannot be, and indeed Conklin has succeeded in shifting it 

 in Crepidula and Cynthia. The polar structure to which it 

 belongs may, therefore, eventually prove to be dependent on 

 'the heteropolar arrangement of certain ooplasmic substances', 

 though these are indistinguishable to the eye, and need not 

 of course be of sufficiently different specific gravities to allow 

 the force applied to overcome their viscosities. 



Taking all the results together, it seems that since it 

 has been demonstrated that the different regions of the 

 cytoplasm do play definite but different parts in development, 

 are in fact determinants of characters which form an integral 

 portion of the total inheritance of the species, the time is 

 ripe for the physical and chemical investigation of the pro- 

 perties of the egg-plasma. 



There remains for discussion Boveri's description of the 

 development of dispermic eggs in A scaris, the history of 

 which shows clearly the influence upon the nucleus of the 

 different regions of the cytoplasm. 



These doubly fertilized eggs divide simultaneously into 

 four cells, arranged in a tetrahedron. 



The subsequent cleavage is of either one of three types 

 according as there are one, two, or three P l cells, each at the 

 end of a T-piece (Fig. 5, p. 13). 



I. One of the four cells divides so that of its two products 

 the outer touches its sister cell but no other cell in the germ. 

 This cell is P l and divides into E M St and P 2 . The three 

 remaining cells are together equivalent to the AB of normal 

 development and give rise to ectoderm. The somatic cells 

 are recognized of course by the diminution of their chro- 

 matin, while the germ-cells have whole chromosomes. 



II. Of the four cells two behave as P lt two as S, and there 



