BEHAVIOR OF THE GAMETES 239 



general contraction of the egg surface. These cortical granules later become 

 merged with the vitelline membrane to form a relatively thick structure 

 (fig. 129). (See Runnstrom, '49.) Fluid collects between the egg surface 

 and the fertilization membrane. 



On the other hand, in the annelid worm, Nereis, there is a complicated 

 reaction at the egg's surface at the time of fertilization (Lillie, F. R., '12). 

 In this egg a definite membrane is present around the newly laid egg. When 

 a sperm has made an intimate contact with the egg's surface, the cortical 

 layer of the egg exudes a substance which passes through the membrane to 

 the outside; this substance turns into jelly on coming in contact with sea water 

 (fig. 130B). The jelly layer carries away the excess sperm from the egg's 

 surface. A striated area then appears between the vitelline membrane and 

 the surface of the egg. This area, shown in fig. 130B as the cortical layer, 

 represents the collapsed walls of small spaces of the superficial layer of the 

 cortex of the egg which exude their contents through the vitelline membrane 

 to form the surrounding jelly. The egg then forms a new ooplasmic surface 

 beneath the collapsed walls of the small spaces of the original cortex (fig. 

 130B, ooplasmic membrane). 



All of these changes and reactions, namely, the formation of the fertiliza- 

 tion membrane, the exudation of cortical granules, and the emission of a fluid 

 or jelly together with the shrinkage of the egg result from changes which 

 occur in the outer layer of the egg's protoplasm or cortex, and consequently 

 may be classified as cortical changes. The activation of the egg at the time 

 of fertilization or during artificial stimulation thus appears to be closely inte- 

 grated with cortical phenomena. It is debatable whether these changes are 

 the result of activation or are a part of the "cause" of activation. 



The particular activity of egg behavior at the time of fertilization which 



Fig. 119. Fertilization phenomena in the egg of Rana pipiens. (Drawings B, D-G 

 made from prepared slides by the courtesy of Dr. C. L. Parmenter.) (A) Semidia- 

 grammatic representation of the egg shortly before ovulation. The germinal vesicle has 

 broken down, and the chromosomes in diakinesis have migrated toward the apex of the 

 animal pole preparatory to the first maturation spindle formation shown in (B). (B) 

 First polar spindle. Tetrad condition of chromosomes in process of separation into the 

 respective dyads. (C) Polar view of egg after first maturation division. Compare with 

 (D), which represents a section of a comparable condition. (D) Lateral view of spindle 

 of second maturation division. First polar body present in a slight depression at animal 

 pole. The egg is spawned in this condition. (E) Second polar body shown in a depres- 

 sion of the animal pole. Within the superficial ooplasm of the egg, the reorganized female 

 pronucleus is shown. (F) Meeting of the two pronuclei is shown in this section of the 

 egg at the bottom of the female copulation path or "egg streak," E.S. (G) Two pronuclei 

 in contact (shown in F) under higher magnification. (H) Entrance and copulation 

 paths of sperm nucleus. (Modified from Rugh: The Frog, Philadelphia, The Blakiston 

 Co., 1951.) (I) Sperm-entrance path, copulation path, and meeting of pronuclei. (From 

 O. Hertwig, 1877.) (J) First cleavage path, showing daughter nuclei. (From O. Hertwig, 

 1877.) (K) External, lateral view of the egg just before first cleavage. Arrows show 

 direction of pigment migration with resulting formation of gray crescent. 



