2 FERTILIZATION 



Godlewski (191 2) made similar observations following heterologous 

 insemination. 



The second phase of fertilization is concerned with the events 

 which take place after the spermatozoon has entered the egg, 

 culminating in the disappearance of the sperm head and the egg 

 nucleus as separate entities. Strictly speaking, therefore, fertiliza- 

 tion begins with the sperm-egg collision and ends with syngamy, 

 the fusion or apposition of the germ nuclei, when the spermatozoon 

 loses its individuality. This series of reactions may take less than 

 an hour; but the student of fertilization inevitably finds himself 

 asking questions about the pre-fertilization behaviour of eggs and 

 spermatozoa, the domain of the gametologist, and about the 

 activity of the egg after syngamy, the domain of the embryologist. 

 Examination of the pre-fertilization behaviour of the gametes must 

 accompany any study of fertilization and this may well seduce the 

 student away from his intractable problem. Mention has been 

 made of spermatozoa colliding with eggs ; why should they be any- 

 where near each other ? Nature answers this question in bewilder- 

 ing and fantastic ways : the archegonia of plants produce chemicals 

 which attract spermatozoa; dogfish and human beings rely on 

 copulation to ensure sperm-egg collisions. Provided there is no 

 moon, the male Platynereis megalops (Verrill) deposits spermatozoa 

 in the mouth of the female, which bursts in about six seconds, 

 liberating fertilized eggs into the sea (Just, 1914). Some further 

 aspects of this problem, the liberation of spermatozoa and eggs in 

 the right place and at the right time, are discussed in later chapters. 



Maturation. The condition of the egg before fertilization, par- 

 ticularly as regards the stage of maturation it has reached, should 

 always be borne in mind when trying to gain some understanding 

 of fertilization. It has been insufficiently emphasized that echino- 

 derm eggs, on which so many experiments have been carried out, 

 are in an exceptional condition from the point of view of matura- 

 tion, at the time of fertilization. Fig. i, which is adapted from 

 Dalcq (1952), explains this point. In sea-weeds, coelenteratcs, 

 and echinoderms, and not all of them, the egg is fertilized after 

 maturation (Class 4 fertilization). In all vertebrates and Branchio- 

 stoma, fertilization takes place at the second maturation meta- 

 phase (Class 3 fertilization), though there is some evidence that 

 fox, dog and horse eggs may be fertilized in the germinal 

 vesicle stage (Pearson & Enders, 1943; van der Stricht, 1923; 



