70 PATTEN. [VoL. XII, 
anterior thoracic appendages; and the posterior infolding, the 
invaginated remnants of the posterior thoracic appendages. 
It is not improbable that Figs. 74 to 78, Pl. VII, represent 
still further degeneration of this kind. 
We cannot be certain whether a common form of embryo 
such as that in Figs. 48 and 94 is due to gradual antero-posterior 
fusion and degeneration up to the fourth thoracic appendages, 
or whether there has been previous fission at that point, fol- 
lowed by the degeneration of the anterior portion. But in 
either case it shows a distinct difference in the vitality of the 
two parts, which confirms our view of their morphological 
difference. 
In Fig. 103 we have the most remarkable example of con- 
striction, followed by antero-posterior degeneration, that has 
been met with. The least modified embryo, A, is a further 
modification of the condition seen in the lower embryo in 
Fig. 98. All the organs in the anterior portion have fused 
leaving a median row of three long appendages, and fairly 
developed cephalic lobes and oesophagus. 
In embryo & the whole anterior portion has degenerated, 
while the closely approximated dorsal organs appear like a pair 
of eyes in front of what is left of the posterior part of the 
thorax. 
In embryo C the same process is carried still further, the dor- 
sal organs now forming a median pit in front of the remainder 
of the thorax, analogous to the median eyes of the cephalic lobes. 
This condition has been reached by all three of the embryos 
shown in Fig. 104. 
The persistence of the posterior part of the thorax, following 
transverse fission, is also well shown in the very advanced 
embryos in Figs. 96,97, and 100. The point where transverse 
constriction has occurred, and the degree of antero-posterior 
degeneration are about the same in each. 
That in cases like those just referred to there may be actual 
separation of the anterior part of the thorax from the posterior 
is shown by Fig. 102, in embryo 2, where in the two widely 
separated portions of the thorax there is nothing left in the 
anterior one except an imperfectly fused pair of appendages. 
