120 PATTEN. 
Fic. 47, X 60, not sectioned. The first and second thoracic segments are 
absent. The cephalic lobes, however, persist as a faint disc of cells without 
character. This case differs somewhat from the preceding ones, in that the effects 
of degeneration do not increase gradually from before backward, for degeneration 
has been greater in the first two post-oral segments than in the cephalic lobes. 
The median space between the appendages of the remaining pairs gradually 
increases toward the posterior end. 
A still further difference between this embryo and the preceding ones is seen 
in the fusion across the median line of the abdominal appendages, the last 
appendage being the smaller one. 
Fic. 48, X 60, sectioned. In this embryo, the sixth pair of thoracic appendages 
are identified by the well-developed flabella. The long median appendage is 
formed by the fusion of the fourth pair of the thoracic appendages, the unfused 
tips of the two original appendages being still visible. The first three pairs 
of appendages and the entire cephalic lobes have disappeared. The nerve- 
cord is normal and well developed from the root of the tail lobe to the base 
of the unpaired appendage of the fourth segment. The abdomen is small, but 
possesses two pairs of normal appendages. There is a very conspicuous tail lobe, 
the size and shape of which suggest the idea that with the diminution in size of 
the embryo, the marginal fold became too large, and the excess accumulated at 
the posterior end to form the tail lobe. Back of the tail lobes ts a small conical 
projection, also visible in the sections, having all the appearance of an unpaired 
thoracic appendage. The presence of an appendage in this place is very remark- 
able, and I am unable to offer any explanation for its occurrence there. 
The body of the embryo forms the floor of a deep depression bounded by the 
marginal fold. The embryo stained very deeply, as it is composed of dense 
tissue containing a large amount of chromatin. Anteriorly, the rim of the 
mesodermic area, which was not visible, or perhaps overlooked in the surface 
views, is easily seen in the sections, apparently preserving its normal size and 
extension for this stage. In the sections that pass a long distance in front of the 
present anterior end of the embryo, the mesodermic rims are seen as widely 
separated as in the normal embryo of this stage. In the line midway between 
them, instead of the nerve-cords and appendages, there is merely a thin, undif- 
ferentiated layer of ectoderm with a few scattering mesoderm cells beneath it. 
Fic. 49, X 60, sectioned. The cephalic lobes and first two thoracic segments 
are absent. No trace whatever of these organs is to be seen in sections, although 
there is a dark area, due to an accumulation of loose cells, where the anterior end 
of the embryo should be. 
The marginal folds, m.f, are distinct and well developed, and extend across 
the median line just in front of the second pair of thoracic appendages. The 
latter are fused with each other at their bases, and just back of them the nerve- 
cord teminates abruptly in a blunt, unpaired process. 
The abdomen is very well developed, and terminates in a broad tail lobe that 
projects upwards and forwards, thus overarching the posterior part of the 
abdomen. 
Such a prominent tail lobe, although occasionally seen, is not a common form 
of abnormality It is suggestive of the prominent tail fold in insects and 
crustacea, but in this peculiar case recalls that seen in scorpion embryos. 
