36 PATTEN. [Vou. XII. 
is difficult to distinguish the telopore from the anal invagina- 
tion, or in other cases from the primitive streak, so great is 
the range and number of modifications that may be observed. 
The infolded abdominal region seems to straighten out in 
some cases, and in the end give rise to a normally segmented 
abdomen. In the majority of cases, however, its presence 
indicates a general weakness that leads ultimately to complete 
degeneration of the whole embryo. In some cases the defect 
persists to very late stages, for trilobite larvae are not uncommon 
that are perfectly formed except for the abdomen, which may 
show any one of the numerous stages of degeneration. 
Whether these defective larvae die, or the abdomen is 
restored after successive moults, is not known. I have looked 
in vain for traces of aborted abdomens and in fact for any kind 
of abnormalities in individuals older than the tribolite stage. 
B. Asymmetry of Abdominal Appendages has been observed 
in a large number of cases. This condition is well shown in 
Figs. 29 and 31, where there are three abdominal appendages 
on the right but no trace of them on the left. In Fig. 29 the 
abdomen is thrown round to the left by the unequal develop- 
ment, and a peculiar, hood-like fold of ectoderm covers its 
posterior portion. 
In Fig. 30, there are four abdominal appendages on the left, 
and only two on the right. In Figs. 32, 37, 38, 53, 54, there 
is strongly marked asymmetry. 
C. Absence of Thoracic Appendages. In the normal em- 
bryos, the second, third, and fourth thoracic appendages appear 
simultaneously, and shortly afterwards in the order named, 
the first, fifth, and sixth. 
In the abnormal forms, any one thoracic appendage, or almost 
any combination of the twelve, may be absent, but, as one might 
infer, from what has been said in reference to the invagination 
of appendages, it is very difficult to determine in any given 
case whether the appendages failed to develop, or whether 
their absence is due to degeneration. 
In order to obtain some light on this point, a dozen or more 
living abnormal embryos were kept under observation from 
the 20th of July till the 17th of August. Most of the eggs 
