214 ADAM SEDGWICK. 



of us assume, that I condemn. It is not the word " cell " which 

 I am at issue with, for structures most conveniently called 

 cells undoubtedly exist, as the ovum, spermatozoon, lymph- 

 cells, &c. ; and I fully agree that the phenomenon called cell- 

 formation is very general in organic life. But at the same 

 time I hold with Sachs and many others that it is not of 

 primary significance, but " merely one of the numerous ex- 

 pressions of the formative forces which reside in all matter." 

 No one who has studied animal tissues could for one moment 

 deny that nuclei have in many cases a relation to the 

 surrounding protoplasm, a relation which is expressed in 

 the arrangement and structure of that protoplasm. They 

 have not always this relation, but it is usually present, and 

 the question is, how are we to interpret it ? That we 

 cannot interpret it finally until we know the relative values of 

 nucleus and extra- nuclear protoplasm, and the functional re- 

 lation between the two, is clear ; but we may form and hold 

 provisional theories. The hypothesis or idea which holds the 

 field at the present day is the cell-theory in its modern form. 

 This theory, recognising the cellular structure (while not ad- 

 miring the phrase, I must use it for want of a better one) 

 asserts that organisms of Metazoa are aggregations or colonies 

 of individuals called cells, and derived from a single primitive 

 individual — the ovum — by successive cell-divisions ; that the 

 meaning of this mode of origin is given by the evolution 

 theory, which allows us to suppose that the ancestor of all 

 Metazoa Avas a unicellular Protozoon, and that the develop- 

 ment of the higher animals is a recapitulation of the develop- 

 ment of the race. Thus the holoblastic cleavage of the ovum 

 represents the process by which the ancestral Protozoon be- 

 came multicellular, and the differentiation of the cells into 

 groups the beginning of cellular differentiation. According 

 to this view the order is : unicellular stage — multicellular 

 stage — differentiation of cells into tissue elements ; cellular 

 structure preceded cell-differentiation, and to get tissues you 

 must first have cells. And ten years ago it Avas commonly 

 held that these cells were primitively separate from one another, 



