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MUSEUM OF COMPARATIVE ZOOLOGY. 77 
ceree has a separate pair of ganglia which ultimately disappear, serving 
only to aid in the formation of the cireumcsophageal commissure. At 
an early stage, then, the nervous elements consist of two rows of ganglia, 
a pair of ganglia for each somite, which are widely separated except in the 
head lobe and the tail lobe, where they are continuous in the median 
line. 
Another important growth on the part of the ectoderm leads to the 
formation of the stomodeum, which arises as a simple tubular infolding 
between the ganglionic thickenings of the cheliceral somites, and imme- 
diately below the ventral margin of the cephalic plate. It becomes ex- 
panded at its deep end into a sort of pocket, but it has only a small 
external opening. The walls of the stomodzum are composed of cells, 
two or three rows deep, which are elongated and somewhat wedge-shaped 
rather than distinctly columnar. 
4, The period of reversion is marked by the origin of many important 
organs: proctodeum, heart, lungs, trachez, spinning glands, muscles, 
etc. The embryo undergoes great changes in external form, gradually 
passing from the condition represented in Pl. II. fig. 8, where the ven- 
tral surface of the embryo is uniformly convex, and occupies an are of 
about 300°, to a form (Pl. II. fig. 11) in which the ventral surface is 
folded upon itself. 
As a prelude to reversion the tail-lobe of the embryo becomes promi- 
nent, being raised from the surface of the egg. The early steps in the 
process of reversion will be best understood from the examination of a 
series of dorsal views. Fig. 13 (Pl. III.) presents the dorsal aspect at 
the beginning of reversion, and Fig. 8 (Pl. II.) a side view at nearly the 
same stage. The tail-lobe has lost its broad rounded character, and is 
being changed into a more distinctively caudate structure. It still re- 
mains nearly in contact with the cephalic lobe. The dorsal elements 
(‘‘terga” of Barrois) have begun their upward growth, and appear in 
the figure as four pairs of prominent lateral elevations. A corresponding 
growth of the abdominal segments is also in progress ; the dorsal elements 
growing upward finally meet in the median line of the back. 
Each of the lobes of the procephalic plate has a semilunar form, and is 
composed of a central area, apparently separated from a marginal rim by 
means of a deep fold (Pl. IV. fig. 23). The prominent upper lip (/r.) 
is apparently an outgrowth of the ventral border of the cephalic plate, 
and overhangs the entrance to the stomodeeum (s d.). 
The cheliceree (1 app.) and the pedipalpi (2 app.) both appear as post- 
oral structures. The prominent ganglia (gz.) belonging to the cheli- 
