58 PATTEN. [Vor. XII. 
Almost any organ of the body, but more especially the brain, 
oesophagus, nerve-cord, abdomen, or appendages, may be absent 
from the start, or may be behind time, or developing normally 
may quickly disappear, without in any case, so far as could be 
learned, either affecting neighboring organs or being affected 
themselves by conditions other than those produced by their 
own growth. 
The variation can be usually traced back to variations in 
half-metameres. This would seem to indicate that the trunk 
of the embryo is a group of integral parts arranged in a double 
series like two rows of segmented animals placed head to head. 
Each half-metamere seems to be endowed at the outset with a 
fixed capital of formative material, which when absent in whole 
or in part, or exhausted, cannot be restored. In addition to 
this the growth of a half-metamere may be hindered or favored 
by local mechanical conditions similar to those producing 
median concrescence and degeneration. 
No other supposition, it seems to me, can explain why one 
leg, for example, out of twelve utterly fails to develop, while 
the rest go on as usual, although all are equally surrounded by 
nourishing yolk and by the same medium. 
With these considerations in mind we can understand how a 
weakening in the developmental forces might be indicated by 
the four following classes of variation, namely: (1) slowness 
of development, (2) small size, (3) absence of organs here and 
there, and (4) gradual reduction of the whole body till it com- 
pletely disappears. 
Let us now consider these four classes in more detail. 
(1) Almost every one of the embryos we have figured is behind 
time, as we have already explained in stating the methods by 
which the material was obtained, and we have also described 
how single organs, such as the appendages, disappear or fail to 
develop, and that the absence of any organ is often followed by 
the complete degeneration of the rest of the metamere. 
(II) One of the most characteristic features of the following 
embryos zs their small size, a fact well brought out by compar- 
ing the figures on Plates I, VI, and VII, all of which are 
drawn to the same scale. In these cases the reduction in size 
