86 



Fig. 2 represents an egg of the clam 

 after the lapse of perhaps two hours from 

 the time it was first brought into contact 

 with the male cell ; it has apparently been 

 impregnated, for the nucleus and nucleo- 

 lus are no longer visible. It has also be- Figure 2. 

 come perfectly spherical and has lost the membranous, 

 iilmy appendage of Fig. 1. 



J^ff-^m^ ¥i<y 3 e^ 

 /^y-j't'^ .;'•••-;,<■ j-.-^aA -^ ^o* ^ ^^ 



^;1>l'o ;;:^\Crf4 when the fi 



exhibits the same egg still later 



first polar cell has appeared as a 

 M minute blister-like jDrominence at one side. 

 P The polar cell is almost transparent and is 

 situated in a slight de|)ression, from which 

 Figure 3. impressed lines radiate, making it appear 

 as if the vitelline membrane were wrinkled in a radial 

 manner. A similar radial wrinkling around the polar 

 area is described by ^V hitman as occurring during the 

 formation of the polar cells of the egg of a small fresh- 

 water leech {Clepsine)^ but it is more strongly marked 

 than in the egg of the clam. I have not noted this radial 

 wrinkling in the egg of the oyster. After a while the 

 egg represented in Fig. 3 undergoes farther change, a 

 series of annular waves pass from the pole of the egg 

 opposite the polar cell and a second polar cell is pushed 

 out, when the radial wrinkles disappear and the polar 

 cells present the appearance of two small spheres joined 

 to each other and attached to the egg by a stalk as in 

 Fig. 5. I should have stated before that the first polar 

 cell originates in the same way as the second — by the 

 passage of a series of w^aves from one jjole to the other. 

 These are not all of the forces that are at work in the 

 extrusion of the polar cell; the nucleus of the egg has 

 been undergoing changes of form, elongating and con- 

 tracting, disap]3earing to assume a spindle form, then 

 contracts into a rounded form, but with one end in inti- 

 mate relation with the polar cells. These changes of 



