52 PRINCIPLES OF EMBRYOLOGY 



contain only the maternal chromosomes and hereditary factors. If the 

 sperm used belonged to a different species to the egg, apparent hybrids 

 appear which, however, have only maternal characteristics. They are 

 knovvn as gynogenetic hybrids. They often survive better than true 

 hybrids between the two species concerned, since their development is 

 not complicated by the presence of the paternal genes which may be in- 

 compatible with the egg cytoplasm. 



It is possible to go further than this, and eliminate not merely the sperm 

 nucleus but the sperm as a whole. Several authors in the eighteen-nineties 

 (Morgan, Hertwig, Loeb) found that ripe eggs of various species could be 

 activated and started on a course of development by purely chemical or 

 physical treatments. In the early years of this century a great deal of work 

 was done on the subject and many different treatments were worked out 

 for different types of eggs. A very large number of agents were found to 

 be effective. For instance temperature shocks, both hot and cold, the 

 action of acids, changes in osmotic pressure, ultra-violet irradiation, 

 physical puncture with the tip of a needle, etc. No one procedure works 

 satisfactorily over the whole range of animal species. 



The development of an egg without fertilisation is knov^ni as partheno- 

 genesis. The procedures for artificial parthenogenesis have in the first 

 place been worked out empirically, as a series of 'cookery book recipes' 

 which experience has shov^oi to be effective in the particular species of 

 animal being studied. There have, of course, been many attempts to 

 formulate a theory which will account satisfactorily for the efTectiveness 

 of the various agents. The most important of these are the following : 



(i) Loeb argued that the most effective procedures involve two stages 

 of treatment. He suggested that the first step is to produce a superficial 

 cytolysis of the egg cortex, which he thought was associated with an 

 increase in respiration. In many species this step can be produced by the 

 action of acids or a temperature shock. The second step is to apply a pro- 

 tective treatment which prevents the cytolysis going too far. Tliis may 

 often be done by treatment with a hypotonic solution. The difficulty with 

 this theory is that the notion of cytolysis is so ill defined as to have little 

 definite meaning. 



(2) F. R. Lilhe ascribed the main role to the operation of fertilizin 

 produced by the egg itself. 



(3) Dalcq, Heilbrumi and Pasteels, emphasised the importance of 

 calcium in the medium, 



(4) R. S. Lillie considers that the parthenogenesis is brought about by 

 an activating agent {A) which is produced from two other substances, a 



