2O6 EDWIN G. CONKLIX. 



perimental work, and by many it was held that the " mosaic 

 theory had received its death blow from the facts of experi- 

 mental embryology." Among the ova in which isolated blasto- 

 meres were found to be capable of complete development were 

 those of ascidians and from this fact it was assumed that here 

 also the early cleavage cells were undifferentiated. 



Nevertheless, although some of the results of experimental 

 embryology seemed to render the mosaic theory untenable, ad- 

 ditionable evidence in favor of it was furnished by another line 

 of work commonly known as "cell-lineage." In a considerable 

 number of animals belonging to distinct phyla (annelids, mol- 

 lusks, polyclades, nemerteans, nematodes, rotifers, crustaceans 

 and ascidians) the cleavage of the egg was found to be constant 

 in form and differential in character and each of the early cleav- 

 age cells was shown to play a perfectly definite part in the build- 

 ing of the embryo. By those who maintained the traditional 

 view as to the simplicity of the egg and the homogeneity of the 

 blastomeres this fact was explained as due to "the continuity of 

 development," "the position of the blastomeres in the cell com- 

 plex," etc. But this explanation was never a satisfactory one 

 and is no longer tenable ; both observation and experiment have 

 shown conclusively that in certain eggs the blastomeres are not 

 all alike. In particular the experimental work of Crampton 

 (1896), Fischel (1897, 1898), Boveri (1901), Wilson (1903, 

 1904), Yatsu Ci9O4) and Zelney (1904) has demonstrated that 

 in ctenophores, echinoderms, nemerteans, mollusks and annelids 

 all portions of the egg are not equipotential ; this, as well as other 

 work on the organization of the egg, proves that there is a differ- 

 entiation and localization of the substances of the egg very unlike 

 the "simple undifferentiated protoplasm" of traditional belief. 



In most instances the protoplasm of the different blastomeres of 

 an egg is much the same in appearance ; in a few cases it is visibly 

 different, but in all cases which have been carefully studied defi- 

 nite blastomeres always give rise to definite parts of the embryo. 

 In fact the manner and rate of development as well as its results 

 are so thoroughly characteristic of certain blastomeres that stu- 

 dents of cell-lineage have usually concluded that the protoplasm 

 of different blastomeres must differ, even though these differences 



