30 HEREDITARY CHARACTERS 



of conjugation may be retarded, apparently indefinitely, by 

 suitable conditions of nourishment and temperature, but 

 nevertheless it appears certain that under ordinary con- 

 ditions conjugation is necessary to the well-being of the 

 race in the case of many unicellular organisms. 



There are, however, many unicellular forms in which 

 no process of fertilisation has been discovered, and this fact 

 suggests very strongly that fertilisation, that is, bi-parental 

 reproduction, is an early product of evolution. 



It has been shown by experiment that in the case of 

 some animals, segmentation of the ovum and the production 

 of an embryo may take place without any fertilisation. Loeb 

 subjected the unfertilised eggs of a sea-urchin to the action 

 of a magnesium salt. 1 The eggs proceeded to segment and 

 produced larvae, and these larvae continued to develop to 

 the pluteus stage, existing for some time as free swimming, 

 independent, and well-developed organisms. This experi- 

 ment shows that fertilisation is not absolutely necessary, 

 even in an organism where parthenogenesis does not occur 

 normally, for the production of a new individual, in this case, 

 at any rate. 



Morgan showed that in embryos produced from one 

 nucleus, only half the number of chromosomes were present 

 and not the full somatic number. 2 This suggests that the 

 full number of chromosomes is not necessary for an organism 

 to develop and to exhibit all its normal racial characters. 



In all cases where fertilisation takes place the main part 

 of the contribution of the sperm is its nucleus. We have 

 already seen that the ovum is generally many hundred, even 

 many thousand times larger than the sperm. At the time 

 of the entry of the sperm into the ovum, the nucleus of the 

 former is, as a rule, very much smaller than that of the 

 latter. A much greater difference in size, however, exists 



1 Loeb, J., "On the Nature of the Process of Fertilisation and the Artificial 

 Production of Normal Larvae," American Journal of Physiology, iii. 3, 1899. 



2 Morgan, T. H., "The Fertilisation of Non-nucleated Fragments of Echino- 

 derm Eggs," Archiv fur Eniwicklungsmcchanik, ii. 2, 1895. 



