154 OHAS. B. WILSON. 



of considerable width. What more natural than that this 

 same activity should manifest itself at the close of segmen- 

 tation in spinning out these cilia ? Evidently they cannot be 

 extrinsic in their origin, but must be intrinsic. Various names 

 have been applied to them, but I would suggest that the 

 term filature expresses better the actual facts in the case — 

 i. e. a spinning out into threads of filose processes. 



These cilia vary greatly in length, the majority of them 

 being far longer than those ordinarily given in published 

 drawings (see 4, p. 72). Furthermore they do not end 

 abruptly at a definite distance from the cell surface, but 

 terminate in fine processes similar to the terminations of the 

 spin-threads from the polar bodies. I have ventured to 

 represent these cilia in one drawing as they really appear 

 (fig. 8), and I believe that such figures reveal their true 

 nature. 



Such a relation between spin-threads and cilia may be 

 taken in evidence to explain the origin of motion in the 

 latter. 



The band of strong cilia around the edge of the oral sur- 

 face are probably the ones first developed. At the close of 

 segmentation there is a well-defined differentiation of cells 

 into ectoderm and entoderm. The former are considerably 

 the smaller, and are more or less cubical in shape, while the 

 latter are elongated into well-defined cylinders (fig. 69). 



During the blastula stage the egg maintains a position in 

 which the entoderm cells occupy the inferior pole, while the 

 ectoderm cells occupy the superior hemisphere and the upper 

 part of the inferior one. The lower surface becomes some- 

 what flattened previous to gastrulation, and it is in the ecto- 

 derm cells bordering this flattened area that cilia are first 

 seen (fig. 5). 



The difficulty lies in proving that these cells maintain their 

 original position through gastrulation. 



Formation of the Grastrula. — Toward the close of the 

 twentieth hour the large entoderm cells at the inferior pole 

 turn inward and upward. The flattened area around this 



