302 THE PHYSIOLOGY OF REPRODUCTION 



the actions of cytolysins generally. The cytolysins present 

 normally in the serum or the cells of one animal are always 

 inactive against the cells of animals of the same species, and act 

 only against cells of animals of a different species. The explana- 

 tion of this fact is to be found, according to Loeb, in the diminished 

 permeability of the cells of one species towards the cytolysins 

 produced by the cells of animals of the same species, the so-called 

 auto-cy toly sins . 



Similarly the ovum of an animal is not permeable to the 

 cytolysins contained in the spermatozoon of an animal of the 

 same species. In order to bring about the development of the 

 mature ovum the auto-cytolysin must be carried bodily into the 

 egg. And that is the function of the motile spermatozoon. 



APPENDIX 1 



More recent experiments on different insects 2 have confirmed 

 the fact that in these animals, as in birds, the main source of 

 the energy which is used up during development is fat. No 

 nitrogen is lost, but some of the protein material undergoes 

 partial oxidation, to uric acid, and may thus contribute to 

 the " energy of development." Whether the glycogen which 

 disappears during development serves as a source of energy 

 is doubtful. The chitin which is deposited in the cuticle of 

 insects is a compound built up mainly of carbohydrate-groups, 

 and it seems likely that these carbohydrate-groups are derived 

 from the glycogen, which thus contributes to the formation of 

 the cuticle. It is interesting to note that glycogen appears to 

 fulfil a similar function in the developing rabbit, where it also 

 contributes to the building up of the growing tissues. 3 It 

 appears indeed to be a general law that carbohydrate material 

 is essential for growth. 



1 See p. 281. 



2 Farkas, toe. cit. ; Weinland, Zeitschr. f. Biologie, vol. xlvii., 1905; 

 vol. xlviii., 1907; vol. li., 1908; vol. Hi., 1909; Tangl, Pfliiger's Archiv, 

 vol. cxxx., 1909. 



3 Lochhead and Cramer, toe. cit. 



