256 RE GENERA TION 



Wilson ('94) has also rejected Roux's hypothesis of qualitative 

 nuclear division, and adopts the view of the totipotence of the early 

 blastomeres. He has also advanced the view that there is during 

 development a progressive differentiation of the cells. In a later con- 

 tribution ('96), he accepts " the view of Hertwig and of Driesch that 

 the various degrees of partial development beginning with the echino- 

 derm egg and culminating in the gasteropod may be due to varying 

 conditions of the egg cytoplasm in the different forms." Wilson 

 points out that the series of forms represented at one end by amphi- 

 oxus and at the other end by the ctenophore and the gasteropod may 

 be brought under a common point of view, " for it is certain that 

 development must be fundamentally of the same nature throughout 

 the series, and the differences must be of secondary moment." 



If we reject, as several students of experimental embryology and 

 of regeneration have done, the Roux-Weismann idea of the existence 

 of pre-formed germs in the nucleus, and also the idea of Hertwig of 

 the equivalency of the first-formed blastomeres, and Driesch's vitalis- 

 tic principle, what position can we take in regard to the problem of 

 development ? We may at least attempt to formulate our present 

 position. 



There must be assumed to exist in the egg an organization of 

 such a kind that it can be divided and subdivided during the cleavage 

 without thereby losing its primary character. The refusion of the 

 cells after each division by means of protoplasmic connections indi- 

 cates how this may be possible. The organization must be thought 

 to be of such a kind that the factors determining the cleavage may 

 be different from those that determine the median plane of the body. 

 This is demonstrated by Pfliiger's experiment in which the position of 

 the cleavage planes is changed, but the embryo appears in relation to 

 the primary meridians. The first-formed blastomeres, that result 

 from the division of the egg, do not seem to be strictly equivalent, 

 but they appear to be in most cases, at least, totipotent. The char- 

 acteristics of each part of the protoplasm may be a factor in determin- 

 ing what sort of structure may come from that part of the egg, but 

 back of this lies the fundamental character of the protoplasm itself, 

 that determines what each part, in its relation to the whole, can do. 

 The division of the nucleus appears to be in all cases an exact quan- 

 titative division, and there is some evidence to show that the early 

 nuclei are all equivalent, or at least totipotent. The division of the 

 protoplasm is often into unlike parts, and the kind of cytoplasm con- 

 tained in a part may or may not limit the potencies of each part. 



One of the most important facts in connection with the organiza- 

 tion is that a part, if separated from the rest, may become a new 

 whole, and this appears to be a fundamental peculiarity of living 



