vi ORGAN-FORMING SUBSTANCES 189 



to allow of them all developing as complete organisms. By the time 

 that the i6-cell stage is reached, however,, successive cleavages have 

 resulted in the cells in the upper pole having so little endoderm-forming 

 in proportion to ectoderm-forming substance, that they are now no 

 longer able to gastrulate, though isolated cells from the lower pole 

 (which contain the. endoderm-forming substance) can still do so as can 

 even a ^V blastomere, if taken from this pole. 



The action of the cytoplasm in determining which cells shall develop 

 into soma and which into gonad in Ascaris (p. 85) is clearly that of an 

 " organ-forming substance." 



It must be granted that the egg cytoplasm contains substances that 

 are necessary to the formation of various tissues and organs. There is, 

 however, no reason to suppose that these substances play an active 

 formative part, or that they are anything other than the conditioning 

 environment or the releasing stimulus through which the nucleus exerts 

 its activities. The external environment of the developing egg contains 

 elements which act in quite as specific a manner as the so-called organ- 

 forming substances in the egg cytoplasm. Thus if Echinoderm eggs 

 are made to undergo their development in water identical with that of 

 sea water, except that it lacks the SO 4 radicle, the gut of the larva is 

 not properly formed ; if the calcium normally present is absent, the 

 blastomeres fail to cohere into a blastula, but fall apart, swim away 

 'by means of their cilia, and eventually die without undergoing any 

 differentiation. Eggs of the Teleostean fish Fundulus heteroclitus, 

 developing in sea water to which MgQ 2 has been added, produce 

 embryos with a single median " Cyclopean " eye, instead of a pair of 

 lateral ones (Stockard, 1909). It would clearly be possible to speak 

 of the SO 4 radicle as a gut-forming substance, of calcium as a blastula- 

 forming substance, and of MgCl 2 as a monocularity-producing body, 

 with as much justification as we call the substances in the egg cyto- 

 plasm which we have just discussed, organ-forming (rather than organ- 

 conditioning) substances. 



The fact appears to be that all these substances, whether within the 

 cytoplasm or without, including yolk and vitelline membrane (the 

 degree of the permeability of which is of vital importance to the embryo), 

 or the relation of the embryo to the mother in viviparous forms, are all 

 alike components of the environment of the morphogenetic factors 

 residing in the nucleus. The fact that certain of them, such as the 

 vitelline membrane, the fine and coarse grains of the yolk, and the 

 " organ-forming substances," are not uniformly distributed, but are 

 more or less localized in various parts of the cytoplasm, does not 

 seem to raise a problem different from that raised by any other adaptive 

 arrangement. 



