214 THE PHYSIOLOGY OF REPRODUCTION 



According to Enriques,^ however, conjugation in Col/poda steini 

 only takes place under certain environmental conditions (e.gi.if the 

 layer of the water is not thicker than two millimetres) and does not 

 occur at all if the conditions of life are stationary, the infusorians 

 going on multiplying indefinitely and the number of divisions 

 from the last conjugation making no difEerence.^ According to 

 Woodruff,^ on the other hand, a varied environment seemed to 

 obviate the necessity for conjugation in Paramosciiim. 



It may seem a far cry from the CiUate Infusorian to the 

 British thoroughbred horse, yet there is evidence that here also 

 an in-bred and relatively infertile race may be rejuvenated 

 through access to new surroundings. AlUson, referring to 

 blood stock of British origin, born in AustraUa and New 

 Zealand, writes as follows ; " We can draw from these, not only 

 strains of blood which we have lost, but horses and mares, 

 born again, so to speak, and admirably suited to strengthen and 

 regenerate our home stock." * The same result is said to have 

 been achieved in the descendants of British horses (especially 

 hackneys) imported into Argentina.^ 



The case of the Porto Santo rabbits and that of the goats 

 of Juan Fernandez, which are cited by Huth ® as evidence that 

 in-breeding is harmless, may perhaps be similarly explained. 



The supposed Chbmotactic Peopeeties of Speematozoa and 

 THEIR Relation to the Phenomena op Feetilisation 



It has been suggested that the spermatozoon is attracted 

 towards the ovum by a chemotactic action which the metabolic 

 products of the latter are able to exert upon the former. 



plants in which the fusing cells or nuclei are closely related. The force of 

 this objection must be admitted. If, however, the conjugating cells have 

 been subjected to slightly different environmental influences, this near 

 relationship is not necessarily a diiBculty. ' Enriqnes, loc. cit. 



' ^ If water from a culture in which conjugation is " epidemic " be added 

 to a normal culture, it is stated to induce conjugation. Conversely, if water 

 from a normal culture is added to a " conjugation culture," it inhibits it. 



' Woodruff, " The Life Cycle of Paramcecmm when Subjected In Varied 

 Environment," Jour, of Exp. Zool., vol. xlii., 1908. 



■* Allison, The British Thoroughbred Horse, London, I90I. 



° Wallace (R.), Argentine Shows and Livestock, Edinburgh, 1904. Cf. 

 also Darwin, Animals and Plants, London, 1905. 



« Huth, The Marriage of Near Kin, 2nd Edition, London, 1887. 



