HABITS, ETC., OP CEREBEATUf-US LAOTRUS. 



153 



are not the same. But it is not unreasonable to suppose 

 that the filaments formed in such stages really do assist in 

 appression, even if we cannot see any motion. A similar 

 function has been pointed out for the connecting filaments 

 formed in Echinoderm eggs (5). It seems probable also that 

 something of this sort produces the spiral movement during 

 the third segmentation. We must admit that some portion 

 of the internal protoplasm possesses contractility, or we are 

 again driven to the improbability that the protoplasm which 

 streams outward to form the spin-threads is radically different 

 from that which remains inside the egg, and that the change 

 takes place at the ectosarc. We thus see that all the facts 

 and inferences to be derived from the visible filose activities 

 give support on the one hand to the hypothesis of fibrillar 

 contractility (van Beneden, 9), and on the other to the ex- 

 planation by means of streaming or osmotic movements of 

 the fluid portions of the protoplasm (Biitschli, 12). Neither 

 of these views can well be excluded in face of the testimony 

 here presented. 



It seems equally impossible to escape the conclusion that 

 both are correct, so far as the existence of the forces which 

 they advocate is concerned. May it not be that we are to 

 look for a full explanation not to either one of them alone, 

 nor to any other single hypothesis already advanced, but 

 rather to a compromise, including at least several of the 

 principal ones ? 



Gastrulation. 



Ciliation of the Larva. — As soon as the blastula stage 

 is reached cilia appear upon the external surface of the cells, 

 and the embryo begins to rotate slowly about a vertical axis. 



The displays of filose spinning already given by the polar 

 bodies and the blastomeres incline me to the view recently 

 expressed (4) that cilia owe their origin to similar activities. 



During segmentation both polar bodies and blastomeres 

 were sending out spin-threads, which often traversed spaces 



