Nost.\] VARIATIONS IN LIMULUS POLYPHEMUS. 4I 
are produced by the usual 
it is more correctly a double rectilinear growth, 
one and a lateral one at 
montuaneles? to sit. vit he 
longitudinal growth may 
bey nepresented “by: the 
lines parallel with A-4, 
and the lateral growth by 
the lines parallel with A-— 
£. The relative age of 
each organ is determined 
by its position in relation 
to these two sets of lines, 
the uppermost @ being the 
oldest and most special- 
ized organ in the body, and 
the lowest A the youngest. 
Now it is obvious on exam- 
ining the diagrams ¢hat the 
median fusion and antero- 
posterior degeneration of 
the organs takes place in 
the reverse order of their 
formation, the oldest dying 
first and the youngest last. 
Considering for the pres- 
ent only the small-lettered 
part of Fig. 1, median 
fusion and degeneration 
such as occurs in Limulus 
will gradually carry the 
hypothenuses of the 
shaded triangles toward 
each other till they meet 
in the median line, the 
shaded areas themselves 
gradually disappearing. 
The half of the embryo 

method of apical growth, or rather 

a longitudinal 
BB 
a8 
SE 
Fic. 2. 
Diagrams to illustrate the laws of growth of segmental 
organs and their disappearance by median fusion. 
