1871.] 241 
[Cope. 
The next grade of animal type is represented by the nucleated cell. 
This is simple in Amaba, complex in Actinophrys, ete. With such forms 
as the latter, cell growth begins, and its development is accomplished by 
cell division. This is simple repetition of ultimate parts. In the growth 
of all higher types, we have nothing more than this, but following a law 
of complex repetition. Thus in the growth of the parts of an archetypal 
vertebral column, or an archetypal limb, we have the repetition of cell 
growth till the first segment is formed, when it ceases at that point, and 
repeats the process again, forming another segment like the first; repeti- 
tion within repetition. So with the construction of muscular tissue; first, 
the nucleated cell repeated in a series, whose adjacent walls disappear, 
and whose cell contents flow together, thus forming a fibrilla; then a rep- 
etition of the same process forming a second fibrilla; and so on to the 
completion of thousands of them in fasciculi. 
Let us then trace the series of repetitions and duplicated and still more 
complex repetitions, seen in following up animal forms from their arche- 
types. 
In the simplest repetition of cell growth in a longitudinal direction we 
have Vidrio; in the centrifugal, Actinophrys. The former may be repre- 
sented by a line of simple dots, thus :—Fig. 1. 
eceeeyales BS covncH DODO 
9 cax00mMtoo0 voeO0mie Go AO. 
4400088 BR 8 cvvveh BSB AZZ. 
In a complex repetition we rarely have the same degree of complication 
in each repeated part. We have it centrifugally almost perfect in a Co- 
lenterate (Actinia) and linearly in some of the lower Entozoa. An arche- 
type of the latter kind might be represented thus :—Fig. 2. In a more 
complex form, as of the proglottides of Tenia, thus :—Fig. 8. The same 
might represent an archetypal vertebrate. 
If now we attempt to express the complication of an organ by modified 
repetition of once identical parts, the history of extremities will serve us. 
Thus the limb of Lenidosiren which is composed of identical segments 
may be thus represented :—Fig. 2. Each longitudinal segment of the 
limb of Ichthyosaurus may be similarly represented with a modification, 
in size only, of the proximal or humerus; thus :—Fig. 4. But in Plesio- 
A. P. §.—VOL. XII—2E. 
