1907.] NATURAL SCIENCES OF PHILADELPHIA. 515 



Previous students of polyclatl embryology have shown the undoul^t- 

 edly spiral nature of the cleavage up until a late stage of segmentation. 

 In this respect the de\'elopment of these platodes corresponds closely 

 with the cleavage of annelidan and molluscan eggs. Wilson (92) says 

 (p. 439): "Up to a late stage in the spiral period (twenty-eight cells) 

 every individual blastomere and every cell division is represented by 

 a corresponding blastomere and a corresponding cell division in the 

 embryo of the polyclad and in that of the gasteropod." The same 

 practically may be said of the annelid. 



This striking resemblance in the early ontogeny of three great groups 

 of animals should not be without some significance. Yet, according 

 to the accounts of these earlier investigators, the later history of the 

 cells in the polyclad embryo differs very greatly from that of the 

 apparently homologous cells in the annelids and mollusks. The 

 difference is so marked that Conklin (97) has characterized it as "very 

 great, perhaps irreconcilable." Wilson (92) has summarized this 

 difference as follows (p. 441): "In the polyclad the first group of 

 micromeres gives rise to the entire ectoblast, the second and third 

 groups to the mesoblast, the macromeres to the entoblast. In the 

 mollusk and annelid, on the other hand, the second and third groups of 

 micromeres give rise to the ectoblast, like the first set, and the meso- 

 derm arises subsequentl3^" 



The formation of the ectoderm from the first quartet alone and of the 

 mesoderm from the whole of the second and third quartets has been a 

 serious stumbling block to those embryologists w^ho have attempted to 

 establish cellular homologies. Wilson in a later paper (94) has cited 

 this case as the climax in the contradictions of comparative embry- 

 ology. A number of recent writers have expressed a doubt as to the 

 correctness of Lang's interpretations. Thus ilead (97) WTites (p. 

 289): "I am not convinced that the cells described by Lang do give 

 rise to the mesoderm, and I believe it possible that the mesoderm of the 

 polyclad is formed in the same manner and from exactly the same cell 

 as in the annelids with unequal cleavage." 



This was the state of affairs until 1898 when Prof. Wilson published 

 in his paper on "Cell Lineage and Ancestral Reminiscences" some 

 observations on a Pacific coast species of Leptoplana. Wilson's 

 investigations showed that Lang was wrong in certain particulars, but 

 did not fulfill Mead's prediction with regard to the mesoblast. Wilson 

 found that each of the first three quartets of micromeres contributed 

 to the formation of ectoderm, but also found that the mesoderm arose 

 by inbudding from cells of the second quartet and possibly some from 



