BEHAVIOR OF THE GAMETES 245 



d. Completion of Maturation Divisions, Ooplasmic Movements, and 



Copulatory Paths of the Male and Female Pronuclei in Eggs 



of Various Chordate Species 



A description of the maturation processes, ooplasmic movements, and the 

 behavior of the male and female pronuclei in the fertilization processes of 

 various chordate species is given below. It should be observed that all of 

 these events occur rather synchronously in the urochordate, Styela, and in 

 the egg of the frog, while in others, such as the prototherian mammal, Echidna, 

 they may come to pass in sequence. 



1) Fertilization in Styela (Cynthia) partita: a) Characteristics of the 

 Egg Before Fertilization. The living, fully formed, primary oocyte of the 

 urochordate, Styela (Cynthia) partita, is about 150 /x in diameter. It possesses 

 at this time three areas which can be distinguished with clearness, namely, a 

 peripheral transparent layer which contains a sparsely distributed yellow pig- 

 ment, a central mass of gray-appearing yolk, and the area of the germinal 

 vesicle, located near the future animal pole of the egg (fig. 132A). 



The first steps leading to the maturation divisions of the chromatin ma- 

 terial take place before sperm entrance, at the time the egg is spawned or 

 shortly before. At this time the wall of the germinal vesicle (i.e., the nuclear 

 membrane) breaks down, and the contained clear cytoplasm moves up to the 

 animal pole of the egg where it spreads out to form a disc. The chromosomes 

 then line up on the metaphase plate of the first maturation spindle; they re- 

 main thus in the metaphase of the first maturation until the sperm enters 

 (fig. 116A, B). 



b) Entrance of the Sperm. The sperm enters the egg (i.e., the primary 

 oocyte) at the future vegetal (vegetative) pole, either exactly at the pole or 

 a little to one side (fig. 116B). Sperm entrance at this pole probably is due 

 to a fundamental structural and physiological condition which in turn reflects 

 a definite polarity of the egg. Only one sperm normally enters the egg, but 

 several sperm may penetrate through the chorion into the perivitelline space. 



c) Cytoplasmic Segregation. A striking series of changes appear within 

 the cytoplasm of the egg, immediately following sperm entrance. The yellow- 

 pigmented, peripheral layer of protoplasm flows toward the point of sperm 

 entrance (i.e., the vegetal pole) and collects into a "deep, orange-yellow 

 spot" which surrounds the sperm (fig. 132B, C, peripheral protoplasm). It 

 later spreads again and then covers most of the lower or vegetal pole of the 

 egg. Accompanying the flow of yellow peripheral protoplasm toward the vegetal 

 egg pole, most of the clear protoplasm of the germinal vesicle (i.e., the nuclear 

 plasm mentioned above) flows with the yellow protoplasm toward the vegetal 

 pole. The clear protoplasm, to some extent, tends to mingle with the yellow- 

 pigmented, peripheral protoplasm. In figure 132C, the clear protoplasm may 

 be observed as a clear area above the yellow-pigmented protoplasm. 



The sperm pronucleus next moves upward away from the vegetal pole and 



