200 WHEELER. [Vo.. IX, 
the nucleus has returned to the resting stage the spermatozoon 
enters the egg. I have several times seen the deeply staining 
and somewhat twisted head of the spermatozoon lying in the 
cytoplasm near the arrested spindle. Further than this I have 
not traced the phenomena of impregnation, as my attention 
was first attracted to them while studying hardened material 
when I was far from the sea-shore. Why a spindle should be 
formed in the mature ovum and no division result, but only a 
return of the nucleus to its resting stage, is not easily under- 
stood. The spindle lies in the centre of the egg and has 
Fic. 2. Egg-string of Planocera inquilina. 
nothing to do with the formation of the polar bodies ; for these 
do not appear till some time after the eggs are laid, as I have 
several times had occasion to observe. Selenka implies that 
the aborting spindle has its vazson d’étre in bringing the 
scattered yolk particles to the center of the egg where they 
belong. In P. zxgutlina this certainly cannot be the object 
of the spindle, for the egg is full of yolk particles and there is 
no noticeable aggregation about the centrally located spindle. 
Lang, who has also observed the aborting spindle in the mature 
uterine eggs of all the Polyclads which came under his notice, 
has objected to Selenka’s view on the very same ground. 
Many specimens of P. zxgudlina deposited their ova in the 
glass dishes in which I kept them from July 28 to the middle 
