NOs i] VARIATIONS IN LIMULUS POLYPHEMUS. 93 
that will under normal conditions produce at the end of a 
certain time an animal of a given form, capable of performing 
a number of activities. The capital each ovum starts with 
determines the result, if the conditions are normal. A change 
in environment may retard or accelerate the mechanism ; it may 
throw certain parts into new tracks ; it may influence the dis- 
tribution of formative material as a whole, but it cannot, within 
the period of one generation, change the nature of the formative 
material. 
However the machine may vary, we recognize it as the same 
machine, — incomplete perhaps, but if so, we recognize the 
vacant places. In very rare cases (only one observed) a Limu- 
lus embryo may have more than twelve thoracic appendages, 
but the extra appendage is exactly like one already existing. 
In no case is there found a new organ or part different 
in kind from those already existing, and in no case is an 
organ, out) of place in reference to others. The chelicerae 
always come back of the cephalic lobes; and a flabellum, 
if it is present at all, always occurs on the outer margin 
of the sixth pair of appendages. We can only attribute 
the original absence of an organ, or of any part of the 
embryo, and the subsequent failure to reproduce that organ, 
to the original absence or diversion into other channels of 
the material out of which that organ was to have been 
formed. 
We can explain the formation of double and triple embryos, 
not on the assumption that the original formative material has 
been increased, but that it has been divided and diverted into 
separate channels, and the consequent diminution in the quan- 
tity available for the new embryos has been one of the causes 
of their subsequent degeneration. 
There are in normal embryos inherent lines of weakness 
along which there is certain to be diminished growth. They 
mark off the different regions of the body from each other; and 
when through the action of the environment or through con- 
genital conditions there is a diminution of vitality, it is shown 
by the diminished size of these regions and by a tendency to 
fuse along the median line. 
