144 Animal Life and hitelligence. 



labour, will become the special business of certain cells. 

 On this view, the specialization of the reproductive faculty 

 and of germinal cells takes its place in line with other cell- 

 differentiations with division of labour ; and the difficulties 

 of comprehending and following the process of differentia- 

 tion in this matter are similar to those which attend 

 physiological division of labour in general. 



It is probable that, in the lower metazoa, in which 

 differentiation has not become excessively stereotyped, the 

 power of developmental reproduction is retained by a great 

 number of cells, even while it is being specialized in certain 

 cells. Hence the ability to produce lost parts and the 

 reproduction of hydra by fission. But, on the other hand, 

 the special differentiation of a tissue on particular lines 

 has always a tendency to disqualify the cells from perform- 

 ing other protoplasmic faculties, and that of developmental 

 reproduction among the number. I do not know of any 

 definite, well-observed cases on record in the animal kingdom 

 of ova or sperms being derived from cells which are highly 

 differentiated in any other respect. In the vertebrata, the 

 mesoblastic, or mid-layer, cells, from which the germinal 

 epithelium arises, have certainly not been previously 

 differentiated in any other line. And in the case of the 

 liydroid zoophytes, quoted by Professor Weismann, the 

 cells which give rise to the germinal products have never 

 been so highly differentiated as to lose the protoplasmic 

 faculty of developmental reproduction. 



Some such view of developmental reproduction, based 

 upon cellular continuity and the division of labour, seems 

 to me more in accord with the general teachings of modern 

 biology than a hypothetical and arbitrary distinction 

 between a supposed germ-plasm and a supposed body- 

 plasm. 



To which category, then, does this hypothesis belong ? 

 Does it support the view that the hen produces the egg or 

 that the ^gg produces the hen ? Undoubtedly the latter. 

 It is based on cellular continuity, and is summarized by the 

 scheme on p. 131. It adequately accounts for hereditary 



