THEORIES OF DEVELOPMENT 247 



of the later stages than that which influences the cleavage. Many- 

 facts of experimental embryology and of regeneration show, more- 

 over, that a new bilateral structure may be readily assumed by 

 pieces that have lost their connection with the rest of the organism. 



After the third division of the egg of the sea-urchin, four of the 

 blastomeres are somewhat different, so far at least as the material of 

 which they are made up is concerned, from the other four ; yet any 

 one of the eight blastomeres, or groups of blastomeres, can produce 

 a whole embryo. The same statement can be made for much later 

 stages, since it has been found that fragments from any part of the 

 blastula wall can give rise to whole embryos, and we may safely attrib- 

 ute this property to all the cells, although on account of the size of the 

 cells of later stages they cannot individually produce a whole embryo, 

 but each can produce any part of an embryo, which amounts to the 

 same thing. If we assume that all of these cells are exactly alike, 

 as Hertwig has done, we fail to see how the next stage in the devel- 

 opment could take place, unless some external factor could act in 

 such a way as to change the different parts of the egg. We have, 

 however, no reason to suppose that all the cells are alike because 

 they are all potentially equal. Even pieces of an adult animal of 

 hydra or of stentor, for example can produce new whole organisms, 

 although we must suppose these pieces to be at first as unlike as are 

 the parts of the body from which they arise. Moreover, we do not 

 know of a single egg or embryo in which we cannot readily detect 

 differences in different parts of the protoplasm. 



Can these gross differences, that we can see, in the materials of 

 the egg explain the different development of the parts of the egg ? 

 It can be shown, I think, that they do not necessarily determine the 

 result. If we cut in two a blastula, so that one piece contains only 

 the cells from the animal half and the other piece cells from the 

 vegetative half, each produces a whole embryo ; yet the one half 

 lacked just those parts which by hypothesis were supposed to 

 determine the gastrulation of the other half. If we suppose that 

 the materials or structures that are characteristic of the vegetative 

 half are gradually distributed from the vegetative to the animal pole 

 in decreasing amounts, then any piece of the egg will contain more 

 of these things at one pole than at the other. If, then, it could be 

 shown that the gastrulation depends on the relative amounts of these 

 materials in the different parts of the blastula, the difficulty met with 

 in the former view disappears in part. I say in part, because the rela- 

 tive amount of materials that produces the results implies a connect- 

 ing substratum that is acted upon and determines the result. Even 

 if we suppose that this polar distribution of material could account 

 for the polar invagination, we should still be at a loss to account for 



