134 THE PHILOSOPHY OF BIOLOGY 



it can be seen that the median longitudinal plane of 

 the embryo does not correspond, except by chance^ 

 with the first division plane. If the ligature be tied 

 tightly, then each of the blastomeres gives rise to an 

 entire embryo. If it is tied in various places monsters 

 of various types are produced. Therefore there is no 

 segregation of the determinants in the first two blasto- 

 meres. These results, moreover, are not exceptional, 

 for similar ones have been obtained with other animal 

 embryos, in fishes, Amphioxus, ascidians, medusae, and 

 hydrozoa, and in some cases even each of the first four 

 blastomeres develops into an entire embryo when it is 

 separated from the rest. In the sea-urchin embryo 

 the blastomeres can be shaken apart ; or by removing 

 the calcium which is contained in sea water the blasto- 

 meres can easily be separated from each other. It 

 was then found by Driesch that each of the blasto- 

 meres in the i6-cell stage could develop into an entire 

 embryo. It is plain, then, that up to this stage at 

 least there has been no segregation of the determinants. 

 Upon the results of these experiments Driesch 

 based his first proof of vitalism. Let us suppose that 

 there is a mechanism in the developing egg. Now the 

 embryo which results from the latter sooner or later 

 acquires a three-dimensional arrangement of parts : 

 head-end differs from tail-end, dorsal surface difters 

 from ventral surface, and the parts differ on either side 

 of the median plane. The mechanism must, therefore^ 

 be one which acts in three dimensions, anterior and 

 posterior, laterally, and dorso-ventrally. We may 

 represent it by a diagram of three co-ordinate axes, 

 X, y, z ; X and y being in the plane of the paper, and z 

 at right angles to the plane of the paper. Now in the 

 2-cell stage the same mechanism must be present, for 

 this stage develops normally into one entire embryo. 



