252 
THEORIES OF HEREDITY. 
Nov.. 1889. 
II., merely as an illustration, the first division is represented 
as producing the precursors, B and C, of two great groups of 
cells, while the second division gives rise to Stage III. in 
which the right and left sides of the body are determined. 
(The hind and front ends of the body are omitted from consi¬ 
deration for the sake of simplicity.) Thus B splits up into 
B1 and Br, which are the precursors of one of the great 
groups on the left and the right sides of the body, respectively. 
C similarly becomes Cl and Cr, the precursors of the other 
great group on either side of the body. One of the latter, Cl, 
carries the unchanged germ-plasm, A, originally derived from 
the ovum, 
At the next division eight cells are produced (Stage IV.), 
and subordinate groups are predetermined on both sides of 
the body. Thus B1 gives rise to two cells, D1 and El, the 
precursors of the two subordinate groups into which the chief 
group afterwards divides on the left side of the body. Br 
similarly divides and initiates the corresponding subordinate 
groups on the right side. Analogous divisions are undergone 
by Cl and Cr. In one of the products of Cl, viz., FI, the 
unchanged germ-plasm is passed on. 
The fourth division produces the fifth and last stage, the 
complete organism. Each of the eight cells again divides, 
producing sixteen cells, arranged as eight pairs. The body 
is, therefore, made up of eight different kinds of somatic 
cells, each kind being represented on both sides of the body. 
The process may be summed up as follows :—In the first 
division. Stage II., one of the two chief groups of somatic 
cells, viz., H, I, J, K, on both sides of the body, was pre¬ 
determined in B ; and the other chief group, L, M, N, 0, was 
predetermined in C. At the second division, Stage III., the 
separation of these two chief groups for the right and left 
sides of the body was determined. B1 became the precursor 
of HI, II, Jl^ and K1 ; Br of Hr, Ir, Jr, and Kr, and similarly 
with Cl and Cr. At the third division, Stage IV., the separa¬ 
tion of the two chief groups into subordinate groups on 
either side of the body was predetermined. Thus B1 gave 
rise to D1 and El, the precursors of the subordinate groups 
HI, II, and Jl, K1 respectively : similarly with the others. 
Finally, in the last division, Stage V.,each subordinate group 
again separates into two kinds of cells, thus making eight 
different kinds altogether. 
This is a very imperfect attempt to realise by an appeal 
to diagrams the course of development in a Metazoon. The 
imperfection lies chiefly in the simplicity of the illustration 
as contrasted with the inconceivable complexity of the actual 
