The Cellular Basis 



143 



1P.S. 



6 



FIG. 46. SECTIONS OF THE EGG OF Styela, showing maturation, fertiliza- 

 tion and early cleavage, i P.S., First polar spindle, p.b., Polar bodies; 

 $ N, sperm nucleus, $ N, egg nucleus, p. I., peripheral layer of yellow pro- 

 toplasm. Cr., Crescent of yellow protoplasm. A s , A a , Anterior cells, 

 B 3 , B a Posterior cells of the 4-cell stage. In I the sperm nucleus and cen- 

 trosome are at the lower pole near the point of entrance ; in 2 and 3 they 

 have moved up to the equator on the posterior side of the egg; in 4 the 

 egg and sperm nuclei have come together and the sperm centrosome has 

 divided and formed the cleavage spindle ; in 5 the egg is dividing into right 

 and left halves ; in 6 it is dividing into anterior and posterior halves. 



