No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 75 
the cerebral hemisphere. It is not probable, therefore, that a 
common median cephalic lobe developed in the same way as a 
common appendage, for the optic ganglion of one side would 
have to take the place of the cerebral hemisphere of the other, 
or vice versa. The different parts are probably formed as 
independent organs in a sequence from the median line later- 
ally, as is the case with the different organs on a metamere. 
That is, a common semicircular lobe is first formed, then the 
cerebral hemisphere, then the optic ganglia, and finally the 
lateral eyes. 
In the earliest stage of a double embryo observed, Fig. go, 
we see how the characteristic median row of unpaired organs is 
forming. The fourth neuromere and the third appendage are 
unpaired, but the tips of the second appendage are just visible 
as two minute papillae, at the summit of a great bilobed 
projection. 
A comparison of this appendage with the third median pair 
in Fig. 91 shows that cach new median appendage divides first 
at the apex, the separation gradually extending toward the base. 
This, tt will be observed, is the exact reverse of what occurs 
when the appendages of the right and left sides unite to form a 
single median one. Compare Figs. 42, 43, 48, and 49. 
In the median line, at the anterior end of the double embryo 
shown in Fig. go, is a small depression in a dark mass of cells. 
The pit probably represents the common Ax/age of the dorso- 
ventral muscles, which are seen to the right and left of each 
head, reaching the surface ectoderm near the apex of the optic 
ganglion. 
In Fig. 91 the separation of the two heads has, by the wedge- 
like ingrowth of the two new halves, been carried down to the 
fifth thoracic metamere. 
There is perhaps an actively growing point at the apex of the 
V, which gradually works backward, thrusting the old halves 
apart. But there is no indication whatever in surface views of 
such a proliferation, for each new part has the appearance of 
being as complete in every detail as the corresponding organs 
in the old halves. 
My sections of double embryos were not perfect enough to be 
