BEHAVIOR OF THE GAMETES 



251 



In demersal eggs, that is, eggs which sink to the bottom, the protoplasmic cap 

 tends to assume an uppermost position. In pelagic eggs, i.e., eggs which float 

 in the water, the protoplasmic disc turns downward since it is the heaviest 

 part of the egg. 



After the polar bodies are given off, the egg-chromatin material reforms 

 the female pronucleus. The latter and the sperm pronucleus migrate to a 

 position near the center of the protoplasmic disc. The first cleavage plane is 

 established within thirty minutes to an hour following sperm entrance. 



5) Fertilization in the Egg of the Hen and the Pigeon. Fertilization in the 

 hen's egg occurs without any demonstrable movement of cytoplasmic mate- 

 rials, as manifested in the eggs of Styela, Amphioxus, frog, and teleost fish. 

 The egg is strongly telolecithal, and the true protoplasm or blastodisc, which 

 takes part in active development, is a flattened mass about 3 mm. in width. 

 The germinal vesicle in the mature egg is approximately 350 ^u in diameter 

 and about 90 /x in thickness (fig. 126A). Approximately 24 hours before 

 ovulation occurs, the wall of the germinal vesicle begins to break down, and 

 the contained nuclear sap spreads in the form of a thin sheet below the 

 ooplasmic membrane overlying the blastodisc (fig. 126B). (See Olsen, '42.) 



Changes in the chromatin material of the germinal vesicle are synchronized 

 with the breakdown of the membranous wall of the vesicle. The chromatin 

 material, extremely diffuse during the period when the yolk material was 

 formed and the egg as a whole was growing rapidly, contracts and assumes 

 the character of thickened chromosomes in the tetrad condition. The diffuse 



FALLOPIAN TUBE 



Fig. 124. Fertilization stages in the rabbit egg. (A, B after Pincus, '39.) (A) Second 

 polar body exuded; male and female pronuclei. (B) Twenty-two hours after copulation, 

 showing two pronuclei close together. (C) Coagulated plug in infundibular portion of 

 Fallopian tube, containing eggs. This plug is dissolved by sperm during fertilization 

 process. 



