CYTOLOGY OF REPRODICTION IN ANIMAL:S 123 



ary oocyte undergoes the second meiotic division, usually at once, forming 

 a second polocyte with one of the daughter nuclei. The other daughtei- 

 nucleus of this division remains in the now mature ovum, or egg. The first 

 polocyte may or may not divide to complete the quartet of cells expected 

 after meiosis. At the close of these meiotic divisions, or maturation 

 divisions, as they are frequently called, the nucleus of the ovum has the 

 gametic number of chromosomes. 



The mature ovum, or female gamete, is bounded by a delicate vitelline 

 membrane and sometimes by additional jelly-like layers. In some ani- 

 mals a thick layer of nutritive albumen is deposited about the ovimi, and 

 around this may later be added further structures, such as the shell 

 membrane and calcareous shell of the bird's egg. The egg of a bird at the 

 time it is laid is therefore more than an egg. The ovum proper has been 

 enormously distended by great amounts of yolk material, its surface mem- 

 brane lying at the outer boundary of this yolk. Most of its protoplasm 

 has taken the form of a flattened yolk-free mass, the g^erminal disc, lying 

 just beneath the membrane at one side (this side lies uppermost in an egg 

 at rest in an incubator). It is surrounded by "white" and a shell which 

 form no part of the egg proper. If the egg is a fertile one, fertilization was 

 accomplished before these modifications and additions appeared, and the 

 development of the resulting embryo has already advanced to the blasto- 

 derm stage (page 128). This development is resumed when the egg is 

 incubated. 



Syngamy. — The term syngamy denotes the sexual union of two 

 gametes, regardless of their relative structure and behavior and the nature 

 of the consequences. When one gamete is large and apparently passive 

 while the other is small and active, the process is referred to as the fertili- 

 zation of one gamete, the egg, by the other, the sperm, since the arrested 

 development of the egg is thereupon resumed. The induction of develop- 

 ment by experimental agencies is accordingly called artificial fertilization, 

 or artificial parth(;nogenesis. Although syngamy and fertilization are 

 often used as interchangeable terms, it should always be remembered that 

 normal gametic union in any case is a mutual reaction and that it is the 

 fusion product that proceeds wdth development. 



Syngamy in most animals includes typically the entrance of the sperm 

 into the egg, a structural transformation of the sperm often accompanied 

 by related structural changes in the egg, a union of the gametic nuclei, 

 and certain physiological alterations in the egg, some of which are 

 initiated even before the sperm's entrance has been completed. In some 

 animals the rhythmic movements of the tail which bring the sperm to the 

 egg continue after the two gametes have come into contact and suggest 

 a boring action instrumental in gametic union. In other cases, however, 

 such movements cease as soon as the sperm reaches the egg membrane, or 



