W. G. VanName — Embryology of JEustylochus. 283 



Van der Stricbt found nine in the polar spindle of Thysanozoon 

 BroccM^ Klinckowstrom and Francotte six in the polar spindle of 

 Prosthecerceus vittatus and Francotte eight in the polar spindles of 

 Leptoplana tremellaris, DUgocladus auritns, Cycloporxis papillosus 

 and Prosthiostomimi sipJmnculus. 



The Second Polar Spindle. 



The centrosomes at the inner pole of the spindle continue to move 

 apart and the centrosphere to become more elongated. Meanwhile 

 the spindle fibers remaining in the egg degenerate and disappear, 

 and the aster fibers also undergo degeneration, becoming fewer in 

 number as well as shorter and less conspicuous. The extent to which 

 this occurs appears to vary greatly in individual eggs but is never, 

 as far as I have seen, carried to complete disappearance. The 

 centrosphere of the inner pole of the first spindle soon begins to 

 assume the form of a spindle (Fig, 14), even while the centrosomes 

 are still quite near together. A light area appears between the cen- 

 trosomes in the elongating centrosphere. This approaches almost to 

 each centrosome and is, as MacFarland (20) found in Piaitlula, 

 without definite outline toward the ends of the incipient spindle. 

 The constriction of the centrosphere mentioned in describing the 

 first polar spindle, has disappeared. 



At a little later stage (Fig. 15) the centrosomes have moved far- 

 ther apart and in the light central area a few fibers may be distin- 

 guished which increase in number and length as the spindle lengthens. 

 The aster rays also increase in number and length as the spindle 

 grows, though they never attain the development found in the first 

 polar spindle. The portion of the old centrosphere surrounding each 

 centrosome assumes the outline and appearance of a complete cen- 

 trosphere, while some of the aster rays, attaching themselves to the 

 chromosomes draw them into the equator and become the mantle 

 fibers. Thus with the exception of the chromosomes, and the outer 

 portions of the aster rays as far as they are built up from the gran- 

 ules of the cytoplasm, the entire mitotic figure appears to be derived 

 from the centrosphere of the inner pole of the preceding spindle. 



This seems enough to prove the incorrectness of calling the cen- 

 trosphere the "centrosom," as MacFarland (20) has done in describ- 

 ing the maturation of the mollusk Diaidula. It is evident that we are 

 not dealing with the centrosome only, but with much else in addition, 

 and that such a use of the term is not in harmony with its definition 

 as a permanent and self-perpetuating organ of the cell. If we are 



