FUETHEE REMARKS ON THE CELL-THEORY. 215 



and that the conaections found between them iu the fully 

 formed tissues were secondary. You had your neuro-epithelial 

 cell, and your musculo-epitlielial cell, each derived from a 

 distinct cell produced by division of the ovum ; and the question 

 was, how do they find each other and become connected?^ 

 Further, in studying the development of a tissue you had to 

 find a group of cells, each of which became modified into one 

 tissue element. Thus the primitive streak was a proliferating 

 mass of ceils which eventually gave rise to a number of meso- 

 dermal tissues ; the nerve-crest similarly was a mass of cells 

 which gave rise to nervous tissues ; a nerve-fibre was one of 

 these cells elongated, and before you would get your nerve-cell 

 and fibre you must have your nerve-crest cell produced by 

 division from the cells of the nerve-cord, and subsequently 

 sending out a process which elongated and travelled to the 

 periphery as a nerve-fibre. 



My work on Peripatus first led me to doubt the validity of 

 this view of the origin of the Metazoon body. In the first 

 place I found that in some forms there is no complete division 

 of the ovum, and on examining the facts I discovered that such 

 forms were more numerous than had beeh supposed. It 

 therefore appeared that in some Metazoa the ovum divided 

 into completely separate cells, while in others it did not so 

 divide. The question then arose, which of these methods is 

 primitive ? and the answer naturally was, the complete division, 

 because this fitted in with our ideas as to the supposed evolu- 

 tion of the Metazoa from a colonial Protozoon. But on 

 reflection this difficulty arose : the individuals of colonial 

 Protozoa are in protoplasmic connection, while the cells of the 

 completely segmenting ova are separate ; so that the parallel 

 between the ontogeny and the phylogeny breaks down in an 

 important particular. To get over this difficulty it was 

 necessary to suppose that the isolation of the segments of 

 incompletely segmenting ova was apparent and not real, that 



> For exposition of this view vide Flemming, ' Zell-Subsianz, Kern u. 

 Zell-Theilung,' Leipzig, 1882, p. 74, and Balfour's Address to the Depart- 

 ment of Anatomy and Physiology at the British Association in 1880. 



