EXPERIMENTAL MORPHOGENESIS 87 



whole subject. Wilson's researches, pursued not only by 

 descriptive methods, 1 but also by means of analytical ex- 

 periment, led him to the highly important discovery that 

 the eggs of several forms (nemertines, molluscs), which 

 after maturation show the mosaic type of specification in 

 their protoplasm to a more or less high degree, fail to 

 show any kind of specification in the distribution of their 

 potencies before maturation has occurred. In the mollusc 

 egg a certain degree of specification is shown already 

 before maturation, but nothing to be compared with what 

 happens afterwards ; in the egg of nemertines there is no 

 specification at all in the unripe egg. 



Maturation thus becomes a part of ontogeny itself; it 

 is not with fertilisation that morphogenesis begins, there 

 is a sort of ontogeny anterior to fertilisation. 



These words constitute a summary of Wilson's researches. 

 Taken together with the general results obtained about 

 the potencies of the blastula and the gastrula of Echinus, 

 they reduce what appeared to be differences of degree 

 or even of kind in the specification of the egg-protoplasm 

 to mere differences in the time of the beginning of real 

 morphogenesis. What occurs in some eggs, as in those of 

 Echinus, at the time of the definite formation of the germ 

 layers, leading to a specification and restriction of their 

 prospective potencies, may happen very much earlier in 

 other eggs. But there exists in every sort of egg an 

 earliest stage, in which all parts of its protoplasm are 



1 Great caution must be taken in attributing any specific morphogenetic 

 part to differently coloured or constructed materials, which may be observed 

 in the egg-protoplasm in certain cases. They may play such a part, but in 

 other cases they certainly do not (see Lyon, Arch. Entw. Mech. 23, 1907). 

 The final decision always depends on experiment. 



