248 FERTILIZATION 



ovulation during its passage through the peritoneal cavity en route to the 

 oviduct (fig. II 9B, C). The secondary oocyte then enters the oviduct, and 

 during its passage posteriad in the latter, the maturation spindle of the second 

 maturation division is formed (fig. 1 19D). The egg is in this condition when 

 it is spawned. Immediately upon its entrance into the water, it is fertilized by 

 the sperm from the amplectant male. 



The sperm enters the egg at a point about 20 to 30 degrees down from the 

 midregion of the animal pole. As it penetrates through the cortex of the egg, 

 a trail of dark pigment from the egg's periphery flows in after the sperm 

 (fig. 11 9H, I). This initial entrance path of the sperm constitutes the pene- 

 tration path. After making its initial entrance, the sperm begins to travel 

 toward its meeting place with the female pronucleus. This secondary path is 

 the copulation path of the sperm (fig. 1191). If the sperm should continue 

 more or less in a straight line toward the egg pronucleus, the penetration path 

 and copulation path would be continuous. However, if the sperm should veer 

 away at an angle from the original penetration path in its journey to meet 

 the female pronucleus, the copulation path would be at an angle to the pene- 

 tration path. 



The second maturation division of the oocyte occurs in about 20 to 30 

 minutes after sperm entrance with a surrounding temperature approximating 

 22° C. After the female pronucleus is organized, it migrates along its copu- 

 lation path toward the meeting place with the sperm pronucleus, located near 

 the center of the animal pole cytoplasm of the egg (fig. 1 19F, G). 



Shortly after the sperm penetrates the egg, it revolves 180 degrees, and 

 the middle-piece area travels foremost. This revolving movement, whereby the 

 middle-piece area assumes a foremost position, is similar to that which occurs 

 in the protochordates, Styela and Amphioxm. This revolving movement ap- 

 pears to be characteristic of all sperm after entering the egg. (See figs. 116, 

 117, 131.) The sperm pronucleus gradually enlarges as it continues along the 

 copulation path, and the first cleavage amphiaster arises in relation to the 

 middle-piece region. 



Fusion of the two pronuclei occurs at about one and one-half to two hours 

 after fertilization at a normal room temperature of about 22 "^ C. (fig. 1 19G). 

 At about two and three-quarter hours after fertilization the first cleavage 

 furrow begins (figs. 119J; 142A). 



As stated above, the peripheral egg cytoplasm with its pigment tends to 

 flow into the interior of the egg, following the trail of the sperm and thus 

 forms a pigmented trail. The migration of the superficial cytoplasm with its 

 pigmented granules is general over the upper pole of the egg and its direction 

 of flow is toward the point of sperm penetration (see arrows, fig. 119K). 

 Consequently, ,at a point on the egg's surface opposite the point of sperm 

 entrance, the peripheral area of the egg becomes lighter in color and assumes 



