No. I.] VARIATIONS [N LIMULUS POLVYPHEMUS. Wes 
lobes. They form a triangular body which grows backwards at 
the apex, and laterally in either direction, along lines parallel 
with the base of the triangle. The two old halves are thus 
wedged apart till, with the complete formation of the new 
halves, the embryos form a straight line, tail to tail. Fig. 92. 


Fic. 7. 
Diagram to illustrate the law of formation of new halves in double embryos. 
The new halves are shaded. 
Each new organ of a metamere appears first as a single organ 
common to both embryos, and having anormal position for each, 
Fig. 7, Additional organs are formed in the same way, in the 
order of their arrangement on the metamere. For example, 
the organ nearest the median line is formed first; this then 
divides into two, and the one lateral to it appears between 
them as a single organ common to both embryos; this divides, 
and the next one appears in the same place, till all the organs 
of a given metamere are formed. The same process takes 
place in the next posterior metamere, but it is always one step 
behind that in the metamere in front of it. 
The result is that in an embryo that has nearly completed 
its division, as in the diagram (Fig. 7), we find a row of 
