176 ESSENTIALS OF BIOLOGY 



After this interchange of nuclei the conjugants separate and 

 each resumes division with normal vigour. 



63c. The Meanings of Conjugation. These are twofold : 



i. Rejuvenation. Senescence is not understood. It is said to 

 be due to {a) progressive increase of nucleoplasm ; {b) progressive 

 increase of cytoplasm ; (c) decreased rate of metabolism ; {d) 

 general lack of balanced activities that are not regulated. These 

 statements mean little. We see, however, that with increase of 

 specialized activities, senescence occurs. On the whole it is in 

 the relatively undifferentiated organisms and cells that mitotic 

 cell division tends to continue indefinitely (but see later in respect 

 of vegetative reproduction). With this slackening of reproductive 

 activity some stimulus to cell division appears to become essential 

 and it is afforded either by some artificial food substance, or 

 chemical change in the nutritive medium, or by conjugation — 

 which brings some foreign substance, or something that sets up 

 readjustment of some kind, into the senescent organism. 



ii. The transition to sex. And certainly conjugation changes 

 the " organization " of the organism. This organization is located 

 in the nuclear constituents of the latter and when two organisms 

 conjugate the nuclear constitution of each is changed. Not only 

 is there a reproductive stimulus but some change in that which 

 confers " characters " on the organism. In the higher organisms 

 such changes in the developmental organization are certainly 

 effected by sexual reproduction. 



In typical conjugation the conjugants are the whole organisms 

 and they are alike in characters. In other cases one of the con- 

 jugants is relatively large and passive, while the other is small 

 and active. In typical sexual reproduction the conjugants (now 

 called " gametes ") are usually markedly different : one (called 

 the ovum) is large and passive and the other (called the sper- 

 matozoon) is small and actively motile. Thus in a typical 

 conjugation among unicellular organisms typical sex is fore- 

 shadowed. 



64. ON REPRODUCTION IN MULTICELLULAR ORGANISMS 



The multicellular organism is also multi-organismal. Its 

 (arbitrary) origin is a single cell, say the fertilized ovum. This 

 reproduces by mitotic division and the result is a great number 

 of originally similar cells which cohere into one assemblage. By 



