Further Examination of the Cell-Theory 221 



Wilson's statciiient already quoted, that "it is a remarkable 

 fact that in a very large number of cases a precise relation 

 exists between the cleavage-products and the adult parts 

 to which they give rise." ^'^ The frog's egg, one of the 

 earliest and most persistently studied of all eggs, was long 

 ago discovered to have certain features about it, even before 

 cell-multiplication set in, that are adumbrative of the struc- 

 tural relations of tlie future embryo and adult. This was 

 partly recognized, as Wilson points out, by Karl Ernst Von 

 Baer, the "father of Embryology" ; and the fact that the 

 first division plane of this egg corresponds with the plane of 

 symmetry of the adult frog was discovered more than a 

 lialf century ago by George Newport. 



Another group of animals in which promorphology in 

 this restricted sense is quite as conspicuous as in the groups 

 already mentioned is the mollusca. Especially note- 

 worthy are molluscs of the octopus kind. A research in 

 this field well known and much admired among embryologists 

 is by S. Watase on the common squid. Wilson epitomizes 

 Watase's results touching this matter as follows : "Here 

 the form of the new-laid egg, before cleavage begins, dis- 

 tinctly foreshadows that of the embryonic body, and forms 

 as it were a mould in which the whole development is cast." "^ 



Watase's own statements are peculiarly instructive since, 

 as is well known to biologists, he has been an extremist on 

 what we might call the aggregative theory of the multi- 

 cellular organism. The fact of a definite organization of 

 the squid in the one-celled stage of its life he fully recog- 

 nizes. This organization is, he says, such "that the plane 

 of the first cleavage furrow may coincide with the plane of 

 the median axis of the embryo, and the sundering of the 

 protoplasmic material may take place into right and left, 

 according to the pre-existing organization of the egg at the 

 time of cleavage ; and in another case the first cleavage may 

 roughly correspond to the differentiation of the ectoderm and 



