No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 99 
cesses cease, not because some hidden spring within needs to 
be renewed, or because, like a spent rocket, it needs to be 
recharged, but because the still perfect mechanism is choked 
with dust and weighed down with uneliminatable material. 
Before the final collapse some virgin fragment of the original 
material escapes to begin the same process anew. 
We need not assume that this new material, or germ, is es- 
sentially different from the living parts of the old machine. 
We certainly need not look on it as a wonderful piece of pyro- 
technics, with countless preéxisting fuses, percussion caps, and 
hidden chambers, adjusted to discharge themselves at the right 
instant and in the proper sequence —a something which needs 
but the spark and afflatus of proper environment to start on 
its heedless career with infinite splutterings and explosions, to 
cease only when it becomes an empty shell! 
There is in our opinion nothing to be gained by such a view 
of developmental processes. Nevertheless, in discussing such 
problems the use of figurative language cannot be avoided, for 
nothing more than a vague conception can be formed of the 
infinitely complex mechanism at work within the embryos even 
of the very lowly organized animals. To follow its normal 
course of action or development to the end, there must be in 
the ovum infinite niceties in the qualitative and quantitative ad- 
justment of the new parts to the old ; the chemical time-locks, 
that are to fix or release, must be set with exactness to the time 
and place, and the whole mechanism made adjustable to a 
rather wide variation in its surroundings. This being the case, 
who will venture to say that the entire absence of the third 
thoracic appendage was due to the fact that a certain particle, 
destined to grow into that particular appendage, was accident- 
ally omitted from the chromatin of the segmentation nucleus ? 
Is it necessary to suppose because the third left thoracic 
appendage in Limulus was invaginated and its mate projected 
freely from the surface, that the former was derived from a 
peculiarly constructed molecule, or biophor, or whatever you 
please to call it, that was bound to develop into an invaginated 
appendage and not one projecting in the normal way? As well 
assume that a river inherits two kinds of sand grains, one of 
