190 MANUAL OF ELEMENTARY ZOOLOGY 



contains cells which will later give rise to germs. Thus 

 the body divides into two parts, but these are alike in 

 composition, each containing both undifferentiated or germ 

 substance and differentiated or body substance. Sexual 

 reproduction is a more complicated process. It involves 

 the formation of reproductive bodies which shall conjugate 

 before they develop. Since conjugation can only take 

 place between single energids, each reproductive body in 

 sexual reproduction must be a single energid, and since this 

 is to reproduce the whole organism, it must not be special- 

 ised for any function within the body. Thus in the forma- 

 tion of the reproductive bodies of sexual reproduction there 

 is a separation of undifferentiated energids from the rest 

 of the body. A reproductive body which consists of a 

 single undifferentiated energid, 1 is known as a germ. Ova 

 and spermatozoa are germs. Usually, but not always, germs 

 must conjugate before they can develop. When they do 

 not conjugate, they have the characters of ova, and the 

 process of reproduction is called parthenogenesis. This 

 occurs in various insects, crustaceans, and worms. Thus 

 in the insect pest of gardens which is known as the green 

 fly or Aphis (p. 289), the female reproduces for several 

 generations without the assistance of males ; and in certain 

 kinds of water-fleas and the minute water animals known 

 as rotifers, fertilisation has never been seen. Partheno- 

 genesis resembles sexual reproduction in being effected by 

 germs, but asexual in not involving conjugation. It is an 



not mean that portions of the protoplasm of a germ — as for instance 

 the tail of a spermatozoon — are not specialised. It does, however, 

 involve the view that some part of the substance of the germ is not 

 specialised in the way in which the substance of the body energids is 

 specialised, but is capable of giving rise to such substance. What this 

 part may be does not certainly appear, though it is often asserted to 

 reside in the nucleus alone. The " immortality " of germs and Protozoa 

 appears to accompany it. It is not necessary to suppose that the differ- 

 ence between the body energids and the germs is brought about by 

 the formation of the former from only some of various kinds of substance 

 possessed by the latter. It may well be due lo the presence of 

 some factor which allows only some of the potentialities inherited from 

 the germ to come to fruition. In this case the apparently equal distri- 

 bution of the germ nucleus during the mitoses of development 

 (p. 106) could be reconciled with the loss of potentialities in the body 

 energids. 



1 Possibly in the male gamete of the ciliata of a nucleus only. 



