72. PATIIEWN. [VoL. XII. 
match each other exactly, and always in the same way, which 
could hardly be the case if two separate embryos had united with 
cach other through accidental contact. 
In Figs. 90 and 91, then, if we have fission instead of fusion, 
one half of each new embryo must come, not from the fission 
of already existing organs, but by the formation of two new 
halves. Where does this new material come from, and by what 
processes of growth are the new halves formed ? 
In answer to the first question we may say at once that 
there is not the slightest evidence of the existence of any 
formative material in the shape of proliferating cells along the 
median line where the new parts are forming. The old halves 
are, to all appearance, separate from the new. As they are 
already specialized, and sharply circumscribed, there seems no 
way open to explain the origin of the new half of a segment 
by lateral budding, or by regeneration, or by growth, from the 
corresponding old one. We might suppose, perhaps, that the 
new neuromeres in Figs. 90 and gi come from a kind of regen- 
eration of the old one, but that could not possibly be the case 
with any of the new organs lateral to the neuromere, such as 
the appendages, sense-organs, and margin of the mesodermic 
area. 
We are left, then, entirely to conjecture as to the origin and 
causation of the new growth. We shall return to this point 
later. 
The new halves are formed, however, in a very definite 
manner, which we shall now proceed to explain. Jz brief, they 
appear in exactly the reverse order of that by which the old ones 
disappear by median fusion ! 
I. DOUBLE EMBRYOS. 
In the formation of double embryos, two new halves 
belonging ultimately to separate embryos, are produced, each 
half being the mirror image of the other, to which it is 
united along its /ateral margin. The new halves first appear 
at the anterior end of the median line of the old embryo, 
probably between the anterior median margins of the cephalic 
