CHAPTER I 



THE MORPHOLOGY OF FERTILIZATION 



Fertilization is the incitement of an egg to development by a 

 spermatozoon, together with the transmission of male hereditary 

 material to the egg. At fertilization the spermatozoon contributes 

 a, the stimulus for development; b, a set of chromosomes em- 

 bodying the paternal contribution to the genetic make-up of the 

 zygote ; and c, a central body which gives rise to, or is concerned 

 with, the machinery for cell division. In some cases the sperma- 

 tozoon, according to its point of entry into the egg, also determines 

 the plane of bilateral symmetry of the embryo. Fertilization is 

 specific and crosses between different sorts of animals are almost 

 always impossible. Apart from a few exceptional cases to be dis- 

 cussed later, fertilization is irreversible. Once an egg has been 

 fertilized, it cannot be re-fertilized, and once an egg has been 

 stimulated to develop parthenogenetically, fertilization cannot be 

 superimposed on parthenogenesis. 



Fertilization can be divided into two phases. The first occurs 

 when the homologous spermatozoon collides with and becomes 

 attached to the egg surface. This is sufficient to set off a train of 

 reactions in the egg which may lead to development. This first 

 phase is called activation and one talks about a spermatozoon 

 activating an egg or an egg being activated by a parthenogenetic 

 agent. The spermatozoa of the worm Rhahditis monohystera 

 Biitschli activate eggs of the same species so that they develop 

 'parthenogenetically', without containing any male hereditary 

 material (Peacock, 1944). This phenomenon is known as pseudo- 

 gamous fertilization and it can be achieved experimentally, by 

 mixing homologous eggs and spermatozoa and separating them 

 after a short time (F. R. Lillie, 19126; Rothschild, 1953), or by 

 heterologous insemination. Bataillon, for example, observed in 

 1909 that the spermatozoa of Triturus alpestris (Laurenti) activated 

 the eggs of Pelodytes punctatiis (Daudin) pseudogamously and it 

 was this observation which led him to carry out his famous ex- 

 periments on the parthenogenetic activation of frogs' eggs by 

 puncturing them with fine glass needles. Both Loeb (1913) and 



