232 INTERNAL FACTORS IV. i 



protoplasm ; near one pole (the animal), and so determining the 

 egg-axis, is the large germinal vesicle. Prior to the extrusion of 

 the polar bodies the latter breaks down and provides a thii'd 

 substance, a clear cytoplasm. On the entrance of the spermato- 

 zoon near the vegetative pole a remarkable change takes place 

 in the arrangement of these materials. The yellow protoplasm 

 flows down to the vegetative pole and arranges itself symmetrically 

 to the axis ; above it is a zone of clear protoplasm derived from the 

 germinal vesicle ; these two areas together occupy about one- 

 third of the egg. The remaining, the animal two-thirds, are 

 occupied by the grey yolk with only a small quantity of clear 

 cytoplasm at the animal pole in which, after the formation of the 

 polar bodies, the female pronucleus is embedded. 



The egg is still radially symmetrical, but by further shiftings 

 of these substances it becomes bilateral. 



The spermatozoon moves in the clear protoplasm to the equator 

 on one side, the posterior side as it will be, and the yellow proto- 

 plasm follows suit; the first is now disposed in a broad equatorial 

 band encircling half the circumference of the egg, the second 

 forms a wide crescentic mass below it and reaches nearly to the 

 vegetative pole. The female pronucleus, with its clear area, now 

 joins the male. 



The egg is now bilaterally symmetrical about a plane which 

 includes the axis, and passes through the middle of the clear and 

 yellow substances and between the two closely apposed pro- 

 nuclei. 



It seems doubtful whether the movement of the clear and 

 yellow material to the posterior side is immediately due to the 

 agency of the sperm, for the latter does not always take the 

 shortest route to the equator; it may enter on what becomes 

 ultimately the anterior side ; and further, when two sperms enter 

 they both travel to the same spot, appearing to be passively 

 carried along by the independent streaming of the egg cyto- 

 plasm. 



The two pronuclei next move into the axis of the egg in the 

 animal hemisphere, together with their clear protoplasm. The 

 grey yolk is thus largely displaced and shifted into the vegeta- 

 tive region. The egg, therefore, now consists of an animal 



