BEHAVIOR OF THE GAMETES 247 



forming aster now rotate 180 degrees, so that the aster hes anterior to the 

 nucleus as it migrates within the egg (fig. 116F, I, J). The sperm aster thus 

 precedes the pronucleus as the latter moves through the cytoplasm (fig. 1 16M) . 



With the movement of the clear and pigmented protoplasmic substances 

 upward toward the equator and to the point marking the future posterior 

 end of the embryo, the sperm pronucleus and aster move upward. This latter 

 movement of the sperm constitutes the copulation path, and it is formed at 

 a sharp angle to the penetration path (figs. 1 16M, 139B). The egg chromatin 

 in the meantime undergoes its first and second maturation divisions (fig. 

 116F-L). After the second polar body has been formed, the haploid number 

 of chromosomes reform the egg nucleus, now called the female pronucleus 

 (fig. 116L, M). The latter then moves downward through the yolk along its 

 copulation path to meet the sperm pronucleus near the posterior pole of the 

 egg (figs. 1 16M-P; 139B). The actual meeting place in the clear cytoplasm is 

 about halfway between the posterior pole and the center of the egg (fig. 139B). 



Shortly before the pronuclei meet, the sperm aster divides, each aster 

 moving to opposite poles of the sperm pronucleus (fig. 116N). The two 

 pronuclei now meet between the amphiaster of the first cleavage (fig. 1 160, P) 

 and thus become enclosed by the amphiaster spindle (fig. 116P). Following 

 this association, the entire complex migrates toward the center of the egg 

 together with a mass of clear cytoplasm. Some of the yellow protoplasm also 

 migrates slightly centerward. The latter movement of the pronuclei toward 

 the center of the egg is called the cleavage path. In the new position, slightly 

 posterior to the egg's center, the pronuclei form an intimate association (figs. 

 11 6P, 139). The chromosomes then make their appearance, the nuclear mem- 

 branes disappear, and the chromosomes line up in the metaphase plate of the 

 first cleavage spindle preparatory to the first cleavage (fig. 116Q). The first 

 cleavage plane always bisects the midplane of the future embryo and hence 

 bisects the yellow and clear protoplasmic crescents (figs. 116R, S; 132F, G). 



2) Fertilization of Amphioxus. The fertilization stages of Amphioxus are 

 shown in figures 117A-I; 139C. The general process of fertilization in this 

 species appears much the same as in Styela. However, in Amphioxus the fer- 

 tilization phenomena cannot be studied as readily for a pigmented material 

 is not formed in the peripheral cytoplasm. According to Conklin ('32), the 

 general movements of the cytoplasmic substances resemble those of Styela. It 

 is to be observed, however, that the copulation paths of the sperm and egg 

 pronuclei, and also the cleavage path of the two pronuclei, are different slightly 

 in Amphioxus from those present in Styela (fig. 139B, C). 



3) Fertilization of the Frog's Egg. The egg of Rana pipiens is spherical 

 and approximately 1.75 mm. in diameter as it hes in the uterine portion of 

 the oviduct just before spawning. The size, however, may vary considerably. 

 It has a darkly pigmented animal pole and a lightly colored vegetal pole. The 

 first maturation division occurs when the egg is ovulated or shortly after 



