FERTILISATION 225 



equal to about two diameters of an ovum. Bordet, 1 however, who 

 likewise experimented upon the FucacecT, obtained no evidence of 

 chemotactic attraction, but he found, on the other hand, that the 

 sperms were very sensitive to contact. 



Jennings, 2 in the course of his experiments on the behaviour of 

 the Protozoa, has shown that these organisms will tend to collect in 

 a drop of acid placed in water. This is due to the fact that, whereas 

 no reaction takes place when the individuals pass from water to 

 acid, there is a distinct reaction in passing in the reverse direction. 

 All the organisms which enter the drop of acid remain there, and 

 consequently they accumulate, but this is not due to any attractive 

 influence on the part of the drop. It is of course possible that 

 Pfeffer's observations on the supposed attraction possessed by 

 malic acid for the spermatozoa of ferns is susceptible of a similar 

 explanation. 



Buller, 3 who has discussed the question at some length and has 

 performed numerous experiments, states that, so far as he is aware^ 

 not a single case is known where chemotaxis plays a part in the 

 fertilisation of the ova of animals. 



CHILD'S THEORY OF THE LIFE CYCLE* 



Child, 5 as the result of some twenty years' study of the process 

 of regeneration in many animals, and especially with regard to 

 Planarians, has brought forward a theory of the life cycle in plants 

 and animals, which is of remarkable interest on account of its 

 many applications. In this theory an attempt has been made to 

 break away completely from the corpuscular conception of the 

 development and inheritance of the organism, which in the past 

 thirty years has dominated all our outlook on this subject. In 

 the following remarks this theory can only be considered in relation 

 to the origin of the gametes, fertilisation, and the problem of the 

 life cycle, very briefly. 



In the animal kingdom there are two well-established means of 

 reproduction, which in many groups such as the Hydroinedusn- 

 alternate with one another, or entirely replace each other, as in 

 some of the lower worms. First, we have asexual reproduction 



1 Bordet, "Contribution a 1'Etude de I'lrritabilite des Spermatozoides chez 

 les Fuccacees,"7.//. (!> t'An/. Iblfjique, vol. xxxvii., 1894. 



2 Jennings, "Studies of Reactions to Stimuli in Unicellular Organisms," 

 .!///</. Jmir of Pkytiol., vol. xxi., 1897. 



3 Buller, " Is Chemotaxis a Factor in the Fertilisation of the Eggs of 

 Animals r : <J/-. ./</<//. .)//</. >W<'rc', vol. xlvi., 1902. 



4 By C. Shearer. 



5 Child's views have been put forward in Senescence and Rejuvenescence, 

 Univ. Chicago Press, 1915 ; also in a smaller work, Individuality in Organisms, 

 Chicago, 1915. To these the reader is referred for further information. 



8 



