218 COMPARATIVE ANATOMY 



feebled powers of the organism. They are the survivors, whilst 

 the somatic cells perish. 



By following up this course of reasoning we can understand 

 why it is that in the higher animals, the Metazoa, there is so 

 close a connection that it amounts to identity between the 

 processes of conjugation and reproduction. 



It has been shown that in all essential respects conjugation 

 is the same thing as fertilisation : the essential act is the ming- 

 ling of nuclear material of two separate individual cells. 



It has further been shown that a recurrence of conjugation 

 (or fertilisation, which is the same thing) at no long intervals 

 of time is essential to the maintenance of the vital activities. 



If then in a cell-composite, or, as we call it, a multi-cellular 

 individual or Metazoon, the capacity for conjugation is restricted 

 to certain cells only viz. to the germ cells it becomes evident 

 that in proportion as organisation advances from the uni-cellular 

 to the multi-cellular condition, and from homogeneity of cell- 

 structure to heterogeneity of cell-structure, so will the primitive 

 distinction between reproduction and conjugation become less 

 and less, until finally only those cells are reproductive which 

 are capable of conjugating namely, the ova (macrogametes) 

 and spermatozoa (microgametes). 



We learn, then, that the progress from uni-cellular to multi- 

 cellular structure involves a division of labour amongst a com- 

 munity of cells, a differentiation in the first instance into 

 somatic and germ cells. The somatic cells, incapable of 

 conjugation, run through a certain course of vital activity and 

 then wear out and perish. But before they perish they give 

 rise to other cells, germ cells, which are capable of conjugation. 

 When two of these have united, the product, a fertilised ovum, 

 enters upon a new career of activity, divides, gives rise to a 

 new composite of somatic cells, and the cycle is again repeated. 



The steps through which this condition came to be estab- 

 lished in the course of evolution are indicated by the series 

 Gonium, Pandorina, Eudorina, and Volvox, and much more 

 obscurely by the series Vorticella, Carchesium, Zoothamnium. 



It is not permissible to suppose that either of these series 

 represents the actual ancestral form of the Metazoa (see p. 186), 

 but they enable us, with good reason, to trace the possible 

 course of evolution from one great division of the animal 

 kingdom to the other. 



