116 PATTEN. 
The embryo is probably in a state of incomplete longitudinal fission, or a 
double embryo, similar to those in Pl. IX, Fig. 98. The two problematical 
appendages, x and y, would then represent the medianly fused, posterior thoracic 
appendages of a second embryo, whose axis is in the line 4. The two embryos 
possess an abdominal nerve-cord in common. The abdominal appendages on the 
left side of embryo & are absent (its right abdominal half was not formed at all 
according to this supposition), and in embryo 4 the thoracic appendages failed to 
develop after the longitudinal fission of the embryo. 
This explanation, requires us to assume nothing more than what we know 
takes place in the double embryos described in plates IX and X. If it is 
correct, this embryo, although resembling that in Fig. 36, owes its present condition 
to a totally different sequence of events. 
Fic. 38, X 33, unsectioned. A well-advanced embryo, the right half of which 
is complete and perfectly normal, except in its slight curvature to the left. Of 
the left half nothing remains but a dark band of inner-layer cells and a small 
posterior appendage situated in a rather deep depression. The long axis of the 
mouth is rotated nearly 45°, so that the apex of the rostrum is thrown toward the 
left. 
