242 INTERNAL FACTORS IV. i 



animal cell gives rise to a larva without an archenteron, an 

 isolated vegetative cell to one in which the apical organ is 

 lacking. So in Amphioxu* \ and \ blastomeres develop into 

 whole embryos, but | blastomeres, whether of animal or vegetative 

 origin, will not gastrulate. In other cases, however, the potentia- 

 lities do not become restricted so soon. Either the four animal 

 or the four vegetative cells of a Frog's egg will give rise to an 

 embryo in which the blastoporic lip and archenteron are de- 

 veloped, while in Echinoderms the least blastomere that will 

 gastrulate is ^. It is not, nevertheless, every cell of the 

 32-celled stage that is capable of developing an archenteron, 

 but only those, as Driesch has conceded, of vegetative origin. 



But whatever differences there may be in the potentialities of 

 the cells of different ova, or of the same ova at different stages 

 or in different regions, it is clear that sooner or later the capacity 

 of a part to become a whole is lost ; and it seems only reason- 

 able to conclude that this loss of totipotentiality is due to the 

 loss of one or more necessary cytoplasmic substances, to the lack 

 of specific material, and not merely of material. In the egg of 

 Ampkioxus, which is telolecithal, there is an obvious difference, 

 in the amount of yolk and size of the granules, between the 

 animal and vegetative hemispheres ; a difference in the capacities 

 of these two regions has been experimentally demonstrated in 

 the Nemertine egg ; and, although it may often appear homo- 

 geneous, the ovum of the sea-urchin, in all probability, possesses 

 that '.stratification ' of potentialities, that is of organ-forming 

 stuffs, at right angles to its axis, on which Boveri has so strongly 

 insisted, for Driesch has admitted that vegetative cells 'gastrulate' 

 more readily than animal cells ; it would appear, then, that a gut- 

 forming substance is present, and a mesenchyme-forming substance 

 too, but that the concentration of these substances steadily 

 diminishes from the vegetative to the animal pole. Radially 

 about the axis the distribution of these substances may be taken 

 to be uniform, since meridional fractions of the egg, \, \, f or f 

 or blastomeres, are invariably totipotential. 



The same explanation may be applied to other cases Hence, 

 speaking generally, the limitation of the potentialities of the 

 parts will depend simply on the original distribution of the 



