THEORIES OF DEVELOPMENT 24$ 



formal grounds, but we are not so much concerned with a purely 

 logical hypothesis as with a verifiable one. 



It has been pointed out that the experiment of compressing the 

 egg in different planes that leads to a new distribution of the nuclei 

 is a formidable obstacle to Roux's hypothesis. If the nuclear divi- 

 sions in the compressed egg are of the same sort as in the normal 

 egg, we should expect to find as a result either a monstrous form 

 with all its parts misplaced, or, if the parts are mutually dependent, 

 nothing at all. Roux has attempted to meet this case by supposing 

 that the nucleus itself responds to the change in the protoplasm and 

 alters its divisions in such a way as to send to each part of the com- 

 pressed egg the right sort of material for that part. This means 

 that the nucleus can so entirely change the sequence of its divisions 

 that instead, for instance, of sending to each pole of the first spindle 

 the material of the right and left sides of the body, as it does nor- 

 mally, it may divide under compression in such a way that the 

 material for the anterior half of the embryo is separated from that 

 of the posterior half. That a change involving such a vast number 

 of qualities could take place, as a result of the slight compression on 

 the egg that brings about a change in the position of the spindle, 

 seems highly improbable. It is, of course, not a disproof of the 

 hypothesis to show that it involves very great complications, for the 

 very assumption itself of a qualitative division of the nucleus, in 

 the Roux-Weismann sense, involves us in great complications. 



A more damaging criticism of the hypothesis of a qualitative 

 division of the nucleus is found in an appeal to direct observation, 

 which shows that the chromatin divides always into exactly equal parts. 

 In many cases we know, from the subsequent fate of the cells, that 

 two cells arising from the same cell play very different rdles in the 

 subsequent development, yet the chromatin of the nucleus is always 

 divided equally. 



The development of the isolated blastomeres of the ctenophore 

 egg may seem at first sight to give support to Roux's hypothesis, for 

 in this case the first two cells are completely separated, and yet give 

 rise to half-structures. Crampton's experiments on the eggs of 

 ilyanassa may also appear to be evidence in favor of this view. 

 In fact, however, they give no more support to the idea of a qualita- 

 tive division of the nucleus than they do to that of a qualitative divi- 

 sion in the protoplasm, and there are some further experiments on 

 the ctenophore egg which indicate that it is the latter rather than the 

 former sort of division that takes place. As stated in the preceding 

 chapter, Driesch and Morgan found that, if a part of the protoplasm 

 of the unsegmented egg of the ctenophore is removed, an incomplete 

 embryo develops, although the whole of the segmentation nucleus is 



