BEHAVIOR OF THE GAMETES 235 



(g) The copulation movements of the egg and sperm pronuclei take place. 

 These movements bring about the association of the two pronuclei 

 near the center of the protoplasm of the egg which is actively con- 

 cerned with the cleavage phenomena. 



(h) The pronuclei may fuse to form a fusion nucleus or they may associate 

 less intimately. Regardless of the exact procedure of nuclear behavior, 

 the female and male haploid chromosome groups eventually become 

 associated in the first cleavage spindle to form one harmonious diploid 

 complex of chromosomes, composed (in most cases) of paired chromo- 

 somal mates or homologues. 



(i) The first cleavage plane is established. 



4. Detailed Description of the Processes Involved in 

 Gametic Union as Outlined Above 



a. Separation and Importance of a Protective Egg Membrane, 

 Exudates, etc. 

 The term "fertilization membrane" is applied to the egg (vitelline) mem- 

 brane which, in many species, becomes apparent only at the time of fertili- 

 zation. In many other eggs a definite and obvious vitelline membrane is present 

 before the egg is fertilized and in many respects functions similarly to the 

 more dramatically formed fertilization membrane. Both types of membrane 

 fulfill definite functions during fertilization and early development. The fer- 

 tilization membrane which forms only as a distinct membrane during fertili- 

 zation was observed first by Fol, in the autumn of 1876, in the starfish egg 

 (Fol, '79). In the cephalochordate, Amphioxus, two definite membranes sep- 

 arate from the egg's surface. One membrane forms just before the sperm enters 

 the egg, while the second membrane separates from the egg after the sperm 

 enters. Both membranes soon fuse and expand to a considerable size, leaving 

 a perivitelline space between them and the egg; the latter space is filled with 

 fluid, the perivitelline fluid (fig. 117B-F, I). In the urochordate, Styela, no 

 such membrane arises from the egg's surface, but the chorion previously 

 formed by the follicle cells serves to fulfill the general functions of a fertiliza- 

 tion membrane (figs. 9 IB, 116). In teleost fishes, the egg emits a considerable 

 quantity of perivitelline fluid at the time of fertilization, effecting a slight 

 shrinkage in egg size with the production of a space filled with this fluid be- 

 tween the egg's surface and the zona radiata (fig. 122A-C). The zona radiata 

 thus functions as a fertilization membrane. In the gobiid fish, Bathygobius 

 soporator, the chorion and/or vitelline membrane expands greatly after the 

 egg is discharged into sea water, and an enlarged capsule is soon formed 

 which assumes the size and shape of the future embryo at the time of hatch- 

 ing (fig. 123). (See Tavolga, '50.) In the brook lamprey, according to 

 Okkelberg ('14), shrinkage of the egg at fertilization is considerable, amount- 

 ing to about 14 per cent of its original volume. A slight egg shrinkage with 



