406 THE EVOLUTION THEORY 



at least to the assumption it makes that the germ-plasm is analysed out 

 during the ontogeny ; and something more must be said on this head. 

 I refer to the numerous facts brought to light through the science of 

 developmental mechanics founded by Wilhelm Roux, and particularly 

 to the investigations as to the prospective significance of the segmen- 

 tation-cells of the animal ovum. 



Among these investigations we find experiments in compressing 

 certain eggs (sea-urchin's) in the early stages of segmentation. The 

 blastomeres are prevented by artificial pressure from grouping 

 themselves in the normal manner ; they are compelled to spread out 

 side by side in the same plane. If the pressure is removed, they 

 change their grouping, and yield a normal embryo. I will not here 

 discuss whether these results can only be interpreted as showing that 

 each segmentation-cell has the same prospective significance, and 

 that it is only its relative position which decides what part of the 

 embryo is to be formed from it ; this could not be done without going 

 into great detail ; I therefore assume it to be true, and confine my 

 survey to the second group of experiments, those on isolated seg- 

 mentation-cells. 



It has been shown that in the eggs of the most diverse animals, 

 for instance in the sea-urchin once more, each of the two first blasto- 

 meres, if separated from one another, can develop into a complete larva. 

 Indeed, in the eggs of sea-urchin and some other animals each of the 

 first four, or any of the first eight, blastomeres, and indeed any segmenta- 

 tion-cell during the earlier stages, possesses the power of developing 

 to a certain point, namely, as far as the so-called ' blastula-larva.' 

 This seems to contradict a theory which assumes that the primary 

 constituents become separated in the successive stages of ontogeny. 

 But in the first place the blastomeres of all animals do not behave in 

 this way, and, moreover, the facts can be quite well explained without 

 entirely renouncing the assumption of the segregation of the deter- 

 minant-complexes. It is only necessary to assume that the segmenta- 

 tion-cells, which develop in the isolated condition as if they were 

 intact eggs, still contain the complete germ-plasm, and that the 

 differential segregation into groups of determinants with dissimilar 

 hereditary tendencies takes place later. This would certainly load 

 the theory with further complications, and I shall not enter into the 

 question here, since the facts which we should have to consider are as 

 yet by no means undisputed. 



But in any case the facts of developmental mechanics referred to, 

 which we owe to numerous excellent observers of the last decade, 

 — I need only name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, 



