512 
MR, J. F. BULLA R ON THE DEVELOPMENT 
indicated by the dotted line it would almost exactly correspond with Cymothoa, and 
this drawing out is what would naturally happen if the development proceeded more 
gradually from before backwards as in Cymothoa. In Oniscus (C) the drawing out is 
completed, the embryo grows almost quite round the egg, and the hind-gut (HG) 
appears before the telson is formed. When it does appear (D) it is directed dorsally, 
not on account of any change in its real direction of growth, but because its develop- 
ment has been delayed so long that the embryo has had time to grow nearly round 
the egg, causing the whole of the posterior ventral surface to point dorsally. 
Cymothoa may thus be regarded as a connecting link between forms like My sis, 
in which there is a strong forward bending, and forms like Oniscus, in which there is 
apparently an equally strong backward bending. 
The rudiments of the limbs have appeared as small rounded projections, whose free 
ends point toward the middle line: they are formed of a fold of epiblast, and their 
central part is tilled with a number of cells from the lower layer. Fig. 8 is a 
transverse section through the thorax ; it is slightly oblique, and so does not pass 
through the point of junction of both the limbs with the body. In the median line 
between the limbs a thickened mass of epiblast has appeared from which will be 
developed the ventral nervous system. At this stage it still consists of undiffer- 
entiated epiblast cells, and has not become separated from the external or epidermic 
layer of the epiblast. 
The only other point to be noticed in this section is the increase in the number of 
the cells of the lower layer. Fig. 9 is a longitudinal section slightly to one side of the 
middle line. At the anterior end the cerebral ganglion is very well marked, and 
some fibrous tissue has been formed in it. Immediately behind this is the involution 
to form the mouth and fore-gut ; it has increased considerably in size since the last 
stage. The most interesting feature of this section is the solid rounded mass of 
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lower layer cells (fig. 9, L) situated in the third post-oral segment from which the liver 
will be developed. 
The involution to form the hind-gut has begun, but is less marked than the oral 
involution. Behind it the telson (fig. 9, T) is very distinct. 
The dorsal organ now consists of several layers of cells, and has become circular in 
shape. In its interior a peculiarly-shaped cavity has made its appearance, and it 
rather closely resembles one of the “ blattformige Anhange ” of Asellus. 
In the fourth stage the egg has altered somewhat in shape ; in the last stage it was 
a nearly regular oval, but now the head has become more distinct. The positions of 
the eyes are marked by slight prominences, and the abdominal region has become 
narrower. The limbs can be distinctly seen through the egg-membranes ; they have 
increased a good deal in size, and are now marked by slight constrictions indicating 
the positions of the future joints. 
Fig. 10 is a transverse section through the anterior part of the thorax. In the 
middle line between the appendages (fig. 10, Ap.) the ventral nerve cord (N) is 
