No.1.] VARIATIONS [IN LIMULUS POLYPHEMUS. 37 
either died or became so opaque that after a few days nothing 
was clearly visible in them. It was, however, clearly estab- 
lished, by means of careful drawings made from time to time 
on such eggs as were transparent enough to allow one to follow 
the changes going on within them, that a gradual atrophy of the 
anterior and posterior extremities took place, while the whole 
embryo showed a marked decrease in size. These facts were 
enough to show that progressive degeneration did take place in 
these embryos, and it is very probable that a similar gradual 
atrophy would have taken place in most of the cases of 
abnormal embryos here cited, if they had been allowed to live 
long enough. 
The simplest cases of the absence of thoracic appendages 
are those where one or more appendages are absent on one 
side of the body, as in Pl. IV, Fig. 30, where the right second 
and third are absent. Similar cases are shown in Figs. 13, 14, 
ZOO 32,33; 34, 35, 30, 37, and others: 
These cases are common, but such as that in Fig. 38 are 
extremely rare, only three similar ones having been seen. In 
this embryo the whole of the left cephalic lobe, nerve-cord, and 
mesoderm, — everything, in fact, except what appears to be the 
left sixth appendage, is absent, while on the right side every- 
thing is perfectly normal, except for the slight spiral curvature 
to the left due to the unequal cell stress. 
A very frequent form of abnormality is where there has 
been a nearly bilaterally symmetrical reduction of the anterior, 
or the posterior, end of the embryo, or both. The cases, in so 
far as they affect the abdomen, have already been considered. 
This process carried still farther would affect the reduction of 
the posterior part of the thorax, but that condition seems to 
be comparatively rare. There is on the contrary a tendency 
to leave this region intact, reducing instead the three anterior ° 
segments one by one, from before backwards, producing con- 
ditions like those in Figs. 16, 18, 24, 25, 26, 27. The small 
embryos with but three pairs of appendages represent the last 
stages of the process, and they are so abundant that I at one 
time supposed they might represent a true wauplius stage. It 
may be that similar embryos gave rise to the statements of 
