DEFINITION 



211 



3. Fusion of the gametes or the second stage of the process of fertilization 



4. Detailed description of the processes involved in gametic union as outlined above 



a. Separation and importance of a protective egg membrane, exudates, etc. 



b. Fertilization cone or attraction cone 



c. Some changes in the physiological activities of the egg at fertilization 



d. Completion of maturation divisions, ooplasmic movements, and copulatory 

 paths of the male and female pronuclei in eggs of various chordate species 



1) Fertilization in Styela (Cynthia) partita 



a) Characteristics of the egg before fertilization 



b) Entrance of the sperm 



c) Cytoplasmic segregation 



d) Copulatory paths and fusion of the gametic pronuclei 



2) Fertilization of Amphioxus 



3) Fertilization of the frog's egg 



4) Fertilization of the teleost fish egg 



5) Fertilization in the egg of the hen and the pigeon 



6) Fertilization in the rabbit 



7) Fertilization in the Echidna, a prototherian mammal 



E. Significance of the maturation divisions of the oocyte in relation to sperm entrance 

 and egg activation 



F. Micropyles and other physiologically determined areas for sperm entrance 



G. Monospermic and polyspermic eggs 



H. Importance of the sperm aster and the origin of the first cleavage amphiaster 

 I. Some related conditions of development associated with the fertilization process 



1. Gynogenesis 



2. Androgenesis 



3. Merogony 



J. Theories of fertilization and egg activation 



A. Definition of Fertilization 



The union or fusion (syngamy) of the oocyte or egg (female gamete) with 

 the sperm (male gamete) to form a zygote is known as fertilization. From 

 this zygotic fusion the new individual arises. Strictly speaking, the word fer- 

 tilization denotes the process of making the egg fruitful (i.e., develop) by 

 means of the sperm's contact with the egg, and as such may not always imply 

 a fusion of the sperm with the egg. In certain types of hybrid crosses, such 

 as in the toad egg (Bufo) inseminated with urodele sperm (Triton), egg acti- 

 vation may occur without fusion of the sperm nucleus with the egg nucleus. 

 Ordinarily, however, the word fertilization denotes a fusion of the two gametes 

 (see Wilson, '25, pp. 460-461). 



The word zygote is derived from a basic Greek word which means to join 

 or yoke together. The word is particularly appropriate in reference to the 

 behavior of the nuclei of the two gametes during fertilization. For, during 

 gametic union, the haploid group of chromosomes from one gamete is added 

 to the haploid group from the other, restoring the diploid or normal number 

 of chromosomes. In most instances, each chromosome from one gamete has 

 a mate or homologue composed of similar genes in the other gamete. There- 



