254 ANIMAL BIOLOGY 



by glands in the wall of the oviduct, some time after fertilization 

 has occurred, when the egg is passing down. (Fig. 169.) 



The reactions of the egg cytoplasm that exclude accessory sperm 

 are overshadowed in importance by others which upset the stable 

 equilibrium of the egg and render its surface permeable, so that 

 extensive osmotic interchanges take place between the cytoplasm 

 of the egg and its surroundings. Most often this is visible merely 

 in a shrinkage of the cytoplasm due to loss of water, but sometimes 

 contractions, amoeboid movements, or flowing of special cyto- 

 plasmic materials to definite regions of the egg are visible. In 

 any event it is certain that profound changes occur in the cyto- 

 plasm — its organization as a gamete soon gives place to a reor- 

 ganization that establishes the general outlines of its subsequent 

 development as a new individual. (Fig. 177, A, B.) 



1. Synkaryon 



Turning now to the nuclei, known as male and female gametic 

 nuclei, the union of which to form the single nucleus (synkaryon) 

 of the zygote is the climax of fertilization. Disregarding the flagel- 

 lum of the sperm, which disappears as it enters the egg, we find 

 that the sperm nucleus moves through a quite definite path toward 

 the center of the egg where it is met by the egg nucleus. Both the 

 gametic nuclei now become resolved into chromosomes which lie 

 free in the cytoplasm, while two centrosomes, each surrounded by 

 an aster, appear and take up positions on either side of the chromo- 

 somes to form a typical mitotic figure. The two sets of chromo- 

 somes form an equatorial plate at the center of the spindle, thus 

 establishing at once not only the mitotic apparatus for the first 

 division of the egg, but also the intimate association on equal terms 

 of chromosomes, with their potentialities from the two parents, 

 to form a common structure — the nuclear complex of the new 

 individual. (Figs. 166, 167, I, II.) 



Such are the outstanding facts of fertilization which a host of 

 investigators have brought to light chiefly within the past sixty 

 years. It was not until 1839 that Schwann, with the establishment 

 of the cell theory, recognized the egg as a cell, and sixteen years 

 more before the sperm was similarly understood ; while the first re- 

 alization that fertilization is an orderly fusion of two cells to form 

 one came during the seventies of the past century. Then it be- 

 came evident that in sexual reproduction each individual con- 



