60 PATTEN. [Vou. XII. 
thoracic. For example, three pairs of thoracic appendages, the 
second, third, and fourth, appear first, followed by a very small 
pair in front, the chelicerae, and by two pairs of appendages 
behind ; the most precocious part is therefore the middle por- 
tion. In the abdomen, the second and third pairs first appear, 
followed by the rudimentary chelaria in front, and by the 
remaining appendages behind. Compare Figs. 4, 6, and 7. 
In the degenerate embryos shown in Figs. 10 and 15, the 
only abdominal appendages present appear to be the second 
and third pairs. It is hard to say whether the ones that 
should develop back of them are merely belated or have degen- 
erated, the general impression at first sight being that there 
has been a shortening of the abdomen by suppression of its 
posterior end. However, in more typical cases, the facts 
seem to support a different conclusion. For example, a large 
part of the abdomen may be absent, and in its place may be 
seen either a conspicuous tail-like projection resembling the 
post-abdomen in young scorpion embryos, or an ingrowth into 
the yolk, which varies greatly in size and depth in different indi- 
viduals. If we may use the presence of this depression as an 
indication of the position of the original posterior end of the 
abdomen, z¢ zs obvious that it ts the anterior metameres that 
have disappeared, because we usually find this depression or 
projection, as the case may be, carried forward to a point just 
back of the thoracic appendages. Compare Figs. 8, 13, 16, 20, 
21, 2A, etc: 
In Figs. 29, 30, and 32 the same law is illustrated by a dif- 
ferent class of cases. Here, also, we see that the principal loss 
of material is at the anterior end of the right or the left side 
of the abdomen, not at its posterior end. 
The abdomen, therefore, ike an independent embryo, develops 
zts metameres in a sequence similar to that in the thorax. 
They degenerate from before backwards, and independently of 
degeneration elsewhere in the embryo. 
The cases we have just described show that there are three 
separate points at which backward degeneration may begin, 
namely, at the anterior end of the cephalic lobes, at the anterior 
