Warren Harmon Lewis 535 



tions of the ectoderm at this stage equi-potential or are there only 

 certain areas that have this power? MencFs theory that the ectoderm 

 of a certain head segment has a tendency to lens formation at a certain 

 developmental stage loses all its meaning when we consider that the lens 

 may arise from abdominal ectoderm. The theory, which has been re- 

 cently advocated again by Schaper, that the lens is a modified primitive 

 sense organ, will not hold in view of the fact that ectoderm, taken from 

 over the abdomen of R. sylvatica and grafted on over the optic vesicle of 

 R. palustris (see experiment Xllg^) did not possess at the time of opera- 

 tion the primitive sense organs and yet it gave rise to a lens. Again it 

 seems unlikely that in the several instances in which I have been able 

 to bring about lens formation from strange ectoderm that the optic 

 vesicle should have in each case come in contact with one of these sense 

 organs. And again in such experiments as IV, in which the optic 

 vesicle has never been in contact with the ectoderm which normally 

 gives rise to a lens there is no trace of a rudimentary lens such as Schaper 

 pictures. 



Weismann's doctrine of determinents is entirely in opposition to this 

 correlative character of the origin of the lens which is fatal to the view 

 that embryonic difl^erentiation is brought about through qualitative 

 nuclear division during cleavage or at any later stage. It is evident 

 that the lens is not predetermined in the egg and that no such things 

 as " lens-biophores " can exist in the egg nucleus, otherwise we should 

 expect the lens to be self-differentiative from a specific group of ecto- 

 dermal cells, and from only these cells. The adherents of Weismannism 

 may take the stand that all of the ectodermal cells contain " lens- 

 biophores " which only become active on stimulation by the optic 

 vesicle. Such a 2>osition is not to be seriously considered. If it is true 

 that the lens is a correlative product of the ectoderm and optic vesicle 

 it is very probable that many other organs of the embryo likewise 

 arise from the interaction between two or more tissues or organs or 

 their products. 



The recent works of Yatsu ° on Cerebratulus eggs, and of Wilson '" on 

 Germinal Localization in Dentalium indicates that there are progressive 

 differentiating changes in the unsegmented egg. This progressive differ- 

 entiation is in the cytoplasm. The mode of formation of the lens gives 

 a clue to these changes. It is easy to imagine that the egg before it 

 leaves the ovary is endowed with a comparatively few specific stuffs 



•Biol. Bui., 1904. 



" Jour, of Expt. Zoology, Vol. I, No. 1, 1904. 



