246 INTERNAL FACTORS IV. i 



geometrical pattern of segmentation may be in the eggs of 

 different animals, whether of the same or of different groups 

 (spiral segmentation of the eggs of Molluscs, Chaetopods and 

 Turbellaria, similar segmentation of the eggs of, for example, 

 some Echinoderms, Polyzoa, and Vertebrates), without neces- 

 sarily involving a similarity in the fate of cells which are 

 identical in origin, it will, I believe, be conceded that the factors 

 which are responsible for cleavage and those which determine 

 differentiation are distinct, though the two may, as in the so- 

 called ' Mosaik ' segmentations, coincide : and where, as in 

 Ascidians, the Ctenophora, the Cephalopoda, this coincidence is 

 complete, the symmetry of the egg, the symmetry of segmenta- 

 tion, and the symmetry of the embryo are one. 



The former factors must be sought for in the arrangement of 

 yolk and protoplasm, possibly in the relative strengths of the 

 centrosomes. in surface-tensions, in the pressure exerted by egg- 

 membranes and so forth ; the latter in definite cytoplasmic organ- 

 forming stuffs. 



(4) Like the parts of the egg the parts of the elementary 

 organs of the embryo are at first equipotential, but exhibit 

 a gradual limitation of potentialities as their development 

 proceeds. As Minot puts it, there is a ' genetic restriction ' of 

 capacities. Thus the archenteron of Asterias can form a new 

 terminal vesicle when this has been removed, but only before 

 the outgrowth of the coelom sacs. The anterior half of the newt 

 embryo will develop into a whole embryo when cut off before the 

 medullary folds have arisen, but not after. When one vitelline 

 vein of a chick is destroyed its fellow will form a whole, not a 

 half heart; and Sumner's experiments quoted above have shown 

 that the margin of the Teleostean blastoderm is isotropic. 



(5) There still remains to be noticed a point of great impor- 

 tance. It certainly has not been demonstrated, and it cannot be 

 pretended that there are in the developed egg as many specific 

 substances as there are separately inheritable qualities in the 

 body. On the contrary, the evidence of experiment speaks 

 against such a supposition ; for, as Driesch has urged, were 

 every separately inheritable quality to be represented in the 

 germ by a distinct morphological unit, the equipotentiality of 



