No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 45 
The whole tendency will be then to produce a form like that 
in Fig. 4, where an attempt has been made to illustrate the 
action of growth forces by lines corresponding with the arrange- 
ment of organs. The increase in width of the lateral ends of 
the metameres will produce a constantly increasing tension 
which will find relief, and thus favor still further growth, 
by movements forwards and backwards. The ends of the 
most anterior metameres will thus be forced together along 
the median line, causing the median fusion of organs in front 
of the true anterior extremity of the body. Conditions like 
these have probably caused the fusion of such organs as the 
median ocelli and the olfactory organs of Limulus, and in insects 
the upper lip, which arises, as has been repeatedly shown, from 
the fusion of originally paired organs lying on the very anterior 
margin of the cephalic lobes. (See “Eyes of Acilius.’’) 
At the posterior end of the body, concrescence of the meso- 
dermic area is the result, and the true apex of the body is shut 
off from growth over the surface of the yolk. The new seg- 
ments formed after this period are therefore forced to grow 
vertically upward and forwards. Hence the conical, forwardly 
directed tail seen so constantly in vertebrates and arthropods. 
The segments formed at the apex of this conical tail lobe 
will be produced under conditions very different from those 
found elsewhere, and we can readily see how these conditions 
might not only be the direct cause of the diminution in size and 
fusion of the organs there, but also prohibit the further addi- 
tion of new segments. 
At the head end there is no necessity for sucha form, because 
no new segments are formed there. But there isa gradual thick- 
ening of all the organs along the median line, which tends to 
find relief forwards as well as laterally ; but as the anterior mar- 
gin of the cephalic lobes is, as it were, shut out from the median 
line by the ingrowth of the mesodermic area, its only relief 
isin buckling upward and forwards. Hence the cerebral flexure, 
and the general S-shaped contour of the whole embryo, Fig. 3. 
Concrescence of the margins of the mesodermic area occurs 
in the normal embryos, as shown by the figures in Pl. I. 
