240 BIOLOGICAL LECTURES. 



be generally believed, that the cells possess a high degree of 

 independence. 



And first we turn to the evidence furnished by experimental 

 work upon the spiral type of cleavage. Evidence of this kind 

 is limited to Crampton's experiments upon Ilyanassa ('96), 

 Wilson's upon Nereis ('92), and Fujita's upon Aplysia ('97), 

 which latter form, according to Blochmann's account of the 

 cleavage ('83), adheres strictly to the spiral type. 



Crampton succeeded in isolating various blastomeres of Ily- 

 anassa, some of which continued to develop for a longer or 

 shorter period. As I showed in 1897, he is in error in regarding 

 the larvae resulting from these isolated blastomeres as strictly 

 partial larvae, since, as he himself states, the entodermal blas- 

 tomeres are completely enclosed by ectoderm, which would be 

 impossible without modification of the form of cleavage. In 

 order to accomplish this enclosure the ectoderm cells must cover 

 a greater amount of surface than that normally covered by them, 

 for, in addition to covering the outer side of the large entoder- 

 mal blastomeres, they must also cover that portion of these cells 

 Avhich is normally in contact with the other entodermal blasto- 

 meres. This modification in form after the injury and the 

 complete closure of the blastopore indicates that the mutilation 

 has affected the whole organism and that a partial rearrange- 

 ment has occurred. And even as regards the earlier cleavages 

 of isolated blastomeres, which Crampton has studied in detail, 

 there is evidence in his figures of some degree of modification 

 in the form of cleavage. The fact that the earlier cleavages do 

 continue so nearly as if the rest of the &^^ were present, even 

 though the blastomeres assume a spherical form, seems to me 

 to indicate simply that the form and direction of cleavage are so 

 fixedly determined by heredity that the shape of the cell has 

 little influence upon them. Intercellular protoplasmic con- 

 nections would also tend to keep the form of cleavage constant. 

 But gradually a rearrangement does occur, or complete closure 

 of the blastopore would be impossible. The fact that the pro- 

 totroch is only partial is due to the early differentiation of these 

 cells. It is not proven, moreover, that it would not have been 

 completed if the larvae had lived long enough. 



