246 FERTILIZATION 



toward one side of the egg to a point which marks the posterior pole of the 

 egg and future embryo (fig. 116M). The clear protoplasm and the yellow- 

 pigmented protoplasm move upward with the sperm (fig. 132D). The yellow- 

 pigmented protoplasm at this time forms a yellow crescent just below the 

 egg's equator, and the middle point of this crescent marks the posterior end 

 of the future embryo (fig. 132D, E). A distinct crescent of clear protoplasm 

 appears just above the yellow crescent at this time (fig. 132D-F). The crescent 

 substance is therefore plainly differentiated at once into clear and yellow proto- 

 plasm, which remain distinct throughout the entire development (ConkUn, 

 '05, p. 21). 



The yolk material, which at first is centrally located in the egg, moves 

 toward the animal pole when the clear and yellow-pigmented protoplasms 

 migrate to the vegetal pole. As the yellow and clear protoplasmic crescents 

 are formed, the yolk material moves to occupy its ultimate position at the 

 vegetal pole of the egg (fig. 132D). Later when the first cleavage division 

 occurs, another crescentic area, the gray crescent, appears on the side of the 

 egg opposite the yellow crescent. 



As a result of the segregation of ooplasmic materials, four definite areas 

 are localized: 



( 1 ) a vegetal, yolk-laden area, 



(2) a gray crescent, 



(3) the yellow and clear protoplasmic crescents opposite the latter, and 

 finally 



(4) the more or less homogeneous cytoplasm at the animal pole of the egg. 



The movements of cytoplasmic materials in the cephalochordate, Amphi- 

 oxus, are similar to those in Styela (Conklin, '32). 



d) CopuLATORY Paths and Fusion of the Gametic Pronuclei. The 

 entrance of the sperm into the egg substance, its migratory movements in the 

 ooplasm, its meeting with the egg pronucleus, and final fusion or association 

 of the pronuclei afford an interesting problem. The factors governing the 

 movements of the female and male pronuclei are unknown, although the move- 

 ments in many eggs are spectacular. The movements of the pronuclei in 

 Styela partita offer an excellent illustration of the copulatory migrations of 

 the pronuclei within the cytoplasm of the egg. 



The sperm enters the egg of Styela partita, as stated previously, at the 

 vegetal pole near the midpolar area or a little to one side (fig. 116B). The 

 sperm moves inward through the yellow-pigmented protoplasm and even- 

 tually becomes surrounded with the yellow and clear protoplasms (figs. 132C; 

 116B-F). This initial pathway through the superficial protoplasm of the egg 

 constitutes the penetration path of the sperm (Wilhelm Roux). The sperm 

 head in the meantime begins to swell and becomes vesicular (figs. 116F, J; 

 133B-G, Ascaris). The nucleus and the middle piece of the sperm with its 



