FURTHER REMARKS ON THE CELL-THEORY. B00 
formed tissues were secondary. You had your neuro-epithelial 
cell, and your musculo-epithelial 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 cells 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 been 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 
they were really connected by protoplasmic strands which had 
1 For exposition of this view vide Flemming, ‘ Zell-Substanz, 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, 
