142 GILBERT ©. BOURNE. 
of such universal application as to constrain us to abandon 
that very useful morphological concept—a cell. 
For some years past the study of cells, of their ultimate 
structure, of their chemical and physical properties, of pheno- 
mena which accompany their growth and division, has been 
carried on with a minuteness which a short time ago was 
undreamt of. And attention has been directed, not only to 
the cells composing adult tissues, but in the most marked 
degree to the successive formation of cells from the primitive 
unit, the oosperm, and to the fate which each subsequently 
undergoes in the course of development. In place of the off- 
hand statements of older embryologists, that the ovum divides 
into two, four, eight, sixteen segments, and so forth, we have 
the most accurate and minute accounts of the successive 
formation of cells, of the place which each occupies in the 
developing embryo, of its parentage and of its progeny, and 
of the share taken by the last named in the building up of the 
adult tissues. In short, we have a number of cell-lineages, 
which show that in a number of animals, some of which are 
widely separate from one another, the formation of cells from 
the ovum follows courses which are either identical or so 
closely similar that the differences excite our wonder far less 
than the similarities. So minute are these investigations that 
every karyokinetic figure has been followed in every cell, up 
to a stage where their number becomes bewildering. 
I refer, of course, to the remarkable series of observations 
which were begun by Selenka, Arnold Lang, Hallez, Bloch- 
mann, and others, and have been carried to the highest 
perfection by von Wistinghausen, E. B. Wilson, Heymons, 
and Lillie. 
It would be impossible, in such an essay as this, to deal 
adequately with the results obtained by these authors; and it 
is unnecessary, since their works are within reach of everyone. 
It is enough to say here that a perusal of them does not tend 
to diminish the importance which we have been accustomed to 
attribute to the cell in developmental processes. 
Nothing can be more clear than the fact that, in Nereis or 
