FERTILISATION 207 



their metabolic relations, are transferred to the daughter-cells. This 

 is true of the sexual reproduction of the higher animals, as well 

 as of the asexual reproduction of unicellular organisms ; in the 

 former, however, the metabolism of one cell, the spermatozoon, is 

 by the process of fertilisation combined with that of another cell, 

 the ovum, into a single resultant, the metabolism of- the offspring 

 that arises from the fertilised ovum : the offspring hence possesses 

 the characters of the two parents." ^ 



Morgan, in a work on the "Physical Basis of Heredity,"^ has 

 collected together the evidence that the chromosomes are the bearers 

 of the hereditary factors. He states that the embryological evidence, 

 while making out a strong case, is of itself insufficient to establish it, 

 but taken in conjunction with the genetic evidence derived from the 

 study of the sex chromosomes (see below, Chapter XV.) and 

 chromosome variation forces the conclusion that this view is correct. 

 On the other hand, Morgan recognises the occurrence of what he 

 calls "cytoplasmic inheritance," and says that the reactions by means 

 of which the embryo develops, and many physiological processes 

 are maintained, reside at the time in the cytoplasm " as the embryo- 

 logical evidence seems to indicate." Furthermore, there is also genetic 

 evidence to show that certain forms of inheritance are the outcome 

 of self-perpetuating bodies in the cytoplasm, most of which go 

 under the name of plastids. Eecognition of plastid inheritance 

 carries with it the idea that if there are such self-perpetuating 

 materials in the cytoplasm they will have to be taken into account 

 in any complete theory of heredity. 



Telegony 



It used to be supposed that the spermatozoa of an animal on 

 being introduced into a female of the same kind, besides fertihsing 

 the ripe ova and producing young, were capable of exercising a 

 permanent influence over the mother, and so transmitting certain of 

 their characters, not only to their own immediate offspring, but to 

 the future offspring of the mother by another sire. This phenomenon,^ 



1 Verworn, lac. cit. Cf. Farmer {loc. cit.), who regards the chromosomes of 

 the nucleus as representing primordia, which are responsible for the appearance 

 of the hereditary characters, but need to be supplemented by specific exciting 

 substances which determine what particular potential character shall actually 

 develop. 



^ Morgan, The Physical Basis of Heredity, Philadelphia and London, 1920. 



' The phenomenon was explained by supposing that the young, while still 

 in utero, in some way affected the mother, and this influence was further trans- 

 mitted to the subsequent offspring. It will be seen that this explanation 

 assumes the possibility of the inheritance of acquired characters of which there 

 is little or no evidence. For recent reviews of this question see Morgan, Experi- 

 mentaZ Zoology, New York, 1907 ; Thomson, Heredity, 4th Edition, London, 1920 ; 

 and MacBride, Science Progress, vol. xv., 1921. See also below, p. 209. 



