SOME TEACHINGS OF DEVELOPMENT. 
203 
in the Strobila stage of development of the larger Medusse^ and 
which presents itself in striking contrast to the arborescent repe- 
tition of the corresponding stage of development of the Craspe- 
dote Medusge. In the so-called Strobila the developing indi- 
vidual has become partially separated by transverse constrictions 
into a series of perfectly similar, saucer- shaped segments, each 
of which comprises within itself in a rudimentary form, a repre- 
sentative of all the parts of the originally simple parent or- 
ganism. So long as these remain united the Strobila is a com- 
pound individual constituted by a serial repetition of similar 
parts ; but they soon prove their individuality by breaking away 
from the compound body and from one another, and maintaining 
existence as independent organisms. 
This serial repetition is seen in still more characteristic form 
amongst the Annelids, where almost every one of the many seg- 
ments which compose the body is complete within itself, and 
any one might be taken as a representation of the primarily 
simple embryo which produced them all. And viewing them by 
the light of the illustrations furnished us by the Coelenterata, it 
seems not unreasonable to regard all animals in which such serial 
repetition is found as compound rather than as simple indi- 
viduals — to a certain extent comparable to the condition of the 
united polyps just referred to, but far more closely joined together 
and interdependent ; rendered thereby incapable of maintaining 
themselves as separate organisms, but, on the other hand, able 
the better, by their united powers, to carry on the struggle for 
existence. And the importance of this serial repetition is indi- 
cated by the fact that it is found as the main characteristic of 
the two most highly organised branches of the animal tree — the 
Arthropoda and the Yertebrata, masked though it often is in 
them by that tendency to recoalescence which at first sight seems 
a recurrence to more simple types of existence, but which, when 
attentively considered, proves to be merely the expression of the 
closer union and combination of the separate segments, the 
better to work together for special objects or for their mutual 
benefit. 
I have stated that, once commenced, the process of develop- 
ment of an animal is a continuous one. Nevertheless, it is 
customary to regard it as advancing by separate steps or stages 
— to look upon it as a staircase or ladder, the stairs or rungs 
of which are successively attained, and the number of which 
depends upon the complexity of organisation of the adult indi- 
vidual. The familiar phrases “scale of development,” “scale 
of organisation” (Latin, scalm, a staircase), are a suflicient in- 
dication of the mode in which the subject is generally viewed. 
And although I think that the continuous nature of the process 
