February 24, 1905.] 



SCIENCE. 



285 



cells. Again, in a worm's egg, after its 

 segmentation into sixteen or more cells, we 

 know very exactly how the materials for 

 the head, the segmented tnmk-region, the 

 digestive tract, the muscles and the ganglia, 

 are distributed among these cells. In all 

 such cases the embryo seems comparable to 

 a piece of mosaic-work, each cell apparently 

 having its own inherent particular char- 

 acter, and its own specific role to play. 



These facts place very conspicuously be- 

 fore us a modern form of the problem of 

 preformation which we may conveniently 

 call the problem of 'germinal prelocaliza- 

 tion.' Does this mosaic-like character of 

 the early embryo mean that the cells are in- 

 herently different? Are they in any de- 

 gree individually predestined for their fu- 

 ture development ; and if such be the case, 

 can this predestination be traced back to 

 protoplasmic regions in the egg before it 

 has divided into cells? In other words, 

 does the egg, or does it not, contain pre- 

 localized, predetermined areas that have 

 any necessary or causal relation to the parts 

 of the future embryo? This is the first 

 guise in which the old question of prefor- 

 mation presents itself to us to-day. I ask 

 you to glance at the results of a few very 

 simple experiments designed to test this 

 question. They will give apparently quite 

 contradictory results. 



Experiments on the eggs of certain ani- 

 mals, such as ctenophores or mollusks, 

 seem to" give an unequivocal answer to our 

 questions. If, for example, the cells of 

 the segmenting egg of the mollusk Den- 

 talium or Patella be separated from one 

 another, at the two-cell stage or any later 

 period, they continue to develop and pro- 

 duce living, actively swimming structures; 

 but these creatures are not completely 

 formed whole embryos, but monsters that 

 in many respects resemble pieces of a single 

 embryo (Fig. 1, A). It is true that the 

 wounds usually close and heal; but these 



structures, nevertheless, remain monstrous 

 and defective, and if they are carefully 

 studied it is found that only when taken 

 collectively can they be said to constitute 

 a single whole embryo. The cells are thus 

 proved to be in some measure inherently 

 different, and to this extent the cell-mosaic 

 is shown to be a real mosaic. If we now 

 extend our operation to the undivided egg, 

 a result in harmony with this is reached. 

 If certain portions of the egg of Dentalium 

 be artificially cut off, the remaining por- 

 tion, upon fertilization, regularly gives rise 

 to a defective and monstrous creature that 

 is not a whole embryo, but resembles a piece 

 or fragment of an embryo. It is evident 

 that this experiment seems to show pretty 

 clearly that even before the egg has begun 

 to divide into cells the parts of the future 

 embryo are in some measure definitely pre- 

 localized and predetermined in its different 

 protoplasmic regions; and evidently, if 

 this be the case, we seem further to have 

 good ground for the mechanistic assump- 

 tion that the undivided egg contains some 

 kind of structural or material configura- 

 tion upon which the character of the de- 

 velopment depends. 



But let us not on this account too hastily 

 accept a theory of preformation or pre- 

 localization. Let us first look at the re- 

 sults of an exactly similar experiment per- 

 formed on the egg of certain other species 

 of animals, for example, Amphioxus, a sea- 

 urchin, or a nemertine worm. Separate 

 here the first two or four cells, and each 

 develops, not into an abortive monster, but 

 into a perfectly formed though dwarf larva 

 (Fig. 1, B). Thus it is possible to pro- 

 duce from a single egg from one to four 

 perfect animals; and in case of certain spe- 

 cies (hydromedusjp) it is theoretically pos- 

 sible by a similar method to produce from 

 a single egg as many as eight or even six- 

 teen perfect dwarfs. Again, in some of 

 these cases, for instance in the nemertine. 



