202 
E. A. SCHAFER. 
So?ne Teachings ^Development. By E. A. Schafer, E.B.S., 
Eullerian Professor of Physiology.^ 
I. 
Development is the term applied to the process of evolution of 
an organism from its simplest to its most complex phase of 
existence. 
In all but the very lowest animals the development of the 
individual begins with the ovum or egg, this being the elemental 
portion or cell of the parent animal which is set aside for the 
special purpose of reproduction. In most cases the separated 
cell is incapable by itself of reproducing the parent ; it must first 
blend with a separated part of another parent. This blending 
with the ovum (female reproductive element) of the anthero- 
zoid (male reproductive element) constitutes the process of 
fertilisation. 
The commencement of development follows immediately upon 
the completion of fertilisation. Once commenced the pr cess is 
usually continuous, and being accompanied by a general increase 
in size, the result is the perfect or adult individual. Although 
these two processes (development, ^. increase of structural and 
functional complexity, and growth, i. e. increase of size) generally 
go hand in hand, we must be careful to distinguish between 
them. Eor mere increase of size may often give the appearance 
of greater complexity, when in reality the organism has not 
advanced either in function or in. intimate structure. This is 
the case sometimes when the increase of size takes the form of 
budded outgrowths which are merely repetitions of the structure 
of the parent organism. The common freshwater polyp con- 
stantly presents us with an illustration of this, for its numerous 
buds produce an appearance of complexity which disappears 
when the buds detach themselves as independent organisms. In 
the compound hydroid polyps, in which the buds remain attached, 
the appearance of complexity remains throughout the life of the 
organism, and is accompanied in some by an actual progress in 
functional and structural complexity. 
In the case just alluded to the increase of size accompanying 
development takes the form of an arborescent repetition of all the 
chief parts of which the developing individual is composed. But 
it may instead take the form of a serial repetition such as we see 
* This article contains the substance of the last two of a series of 
twelve lectures on Animal Development, delivered at the Royal Insti- 
tution in Jan., Feb,, March, 1879 ; and has not been modified since that 
date. 
