No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 67 
comparatively late period. This is certainly the case with the 
embryos shown in Figs. 90 and gI, the type most frequently 
seen. 
In the formation of multiple embryos, we may distinguish 
two kinds of fission: 
(1) Transverse fission, dividing the embryo into anterior 
and posterior portions, the plane of fission being usually 
between the third and fourth thoracic appendages. 
(2) Longitudinal fission, beginning at the anterior end. 
This is the most common form, and one presenting a great 
number of modifications through degeneration. The process ts 
essentially different from that of transverse fission, for the latter 
is the result of a local transverse concrescence and degeneration, 
while longitudinal fission consists in the formation of two new 
halves of an embryo along the median line of one already extst- 
ng. The formation of the new halves begins at the anterior 
end and extends gradually backwards, one new half being a 
mirror image of the other. The old halves are thus thrust 
apart, and with the newly formed halves make new embryos} 
This process may be repeated a second time tn one of the new 
embryos, thus producing three embryos, tatl to tatl, consisting of 
the two original halves plus four new ones. 
A. TRANSVERSE FISSION. 
There are more or less clear indications of this form of 
fission in a great many embryos, but it is rarely that it is very 
clearly marked. The fact that the plane of fission occurs at a 
definite point is very remarkable, and indicates a break in the 
morphological continuity of the embryo of considerable theo- 
retical interest. 
In normal embryos, neither the order of development of the 
appendages, nor their size or shape, gives any indication of this 
cleavage line. The great development of the lateral, segmental 
1 Indications of longitudinal fission, beginning at the posterior end and extend- 
ing forwards, are very rare. No dona fide case has been observed, and I doubt 
whether it ever occurs in Limulus, the cases in which it appears to exist being 
perhaps better explained as malformations of the posterior end of the body, rather 
than as the beginning of true fission. 
