34 PATTEN. [Vor X1k 
degeneration more or less complete of all the other organs on 
the same half metamere, but as these organs are less easily seen, 
I have confined my statements in the main to the appendages. 
The degeneration of an appendage is an indication that degen- 
eration has taken place or will take place in the other organs of 
that half-metamere, but the reverse is not true, for an appendage 
frequently persists long after all associated organs have disap- 
peared. Absence of appendages is one of the most common 
abnormalities, and like the invagination of appendages is most 
likely to occur in embryos showing other indications of abnor- 
mality. Zhe three anterior thoracic, first, second, and third, and 
the abdominal appendages are most frequently absent. The three 
posterior pairs of thoracic appendages are found so frequently 
after everything else has disappeared, that one might suppose 
a nauplius-like larva was a normal feature in the development 
of Limulus, but a little consideration will show that such is not 
the case. t) 
While the appendages are frequently absent in pairs, leaving 
bilaterally symmetrical embryos, this is by no means the rule. 
A. Absence of Abdominal Appendages.— This is perhaps 
the most common of all the modifications observed. A great 
many of the abnormal embryos showed some abbreviation of 
the abdomen, accompanied by the reduction or the entire ab- 
sence of the appendages. 
Fig. § shows the condition of the abdomen in normal 
embryos at the time all the thoracic appendages have appeared. 
The primitive streak is still present as a narrow, longitudinal 
furrow, from the floor of which there is an’ inward.proliferation 
Oficells: > 
One frequently finds embryos, otherwise normal, in which 
the marginal fold, f., which represents the margin of the 
future thoracic and abdominal shields, sweeps across the 
median line just back of the first pair of thoracic appendages. 
They are so much like what occurs in Fig. 8, that it was not 
necessary to represent them. In some of these cases, the 
1T have unfortunately not tabulated all the cases seen, but I havea strong 
impression that degeneration is more likely to affect a half metamere than a 
whole one; as though the former were the unit of structure rather than the latter. 
