OF THE PARASITIC ISOPODA. 
513 
seen. The external layer of epiblast cells lias become separated from it everywhere, 
except at the most ventral point. In the dorsal part of the nervous system two 
fibrous cords have been formed ; these are continuous with the fibrous tissue in the 
cerebral ganglion. Longitudinal sections show that the ventral nervous system is no 
longer a continuous mass of cells, but that segmentation has begun at the anterior end 
and is extending backwards. The bodies marked Ap. are portions of the appendages, 
which owing to their oblique position have been cut nearly transversely. Their 
central part consists, as it did in the last stage, of undifferentiated cells. The cells of 
the lower layer are especially abundant in the neighbourhood of the liver, two of 
whose coeca are seen on the right and one on the left hand side of the figure. These 
tubes have been developed from the solid mass of cells seen in the last stage (fig. 9, L). 
Their relations to the other parts of the embryo are best seen in a horizontal 
section, such as that represented in fig. 11 . This section passes through the liver on 
both sides of the animal. Each half of the liver consists of three very short ccecal 
tubes, opening by a common aperture to the yolk, and, at any rate as far as can be 
made out from sections, quite unconnected with each other, and with the fore and 
hind-guts. Between, and a little in front of the liver coeca, the section passes through 
the top of the fore-gut (EG.) Both the fore- and hind-guts have increased a good deal, 
and the fore-gut has become dilated at its posterior end to form the stomach. 
At the sides of the body the epiblast is folded at each segment, and beneath it are 
a quantity of still undifferentiated cells, from which the lateral muscles, &c., will be 
developed. There are also some scattered cells round the liver coeca, and a large number 
near the fore-gut and cerebral ganglion. The large masses of cells at the anterior end 
of the section are parts of the cerebral ganglion. The two of these which are in 
apposition in the middle line, are continuous, on the one hand with the circum- 
oesophageal cords, and on the other, at a point dorsal to that through which the 
section passes, with the lateral masses of the cerebral ganglion. 
The epiblast covering the sides of the head has become split off* from the ganglion, 
and already at this stage is considerably thickened in the position of the eyes. 
Before going on to the next stage it will be best to notice shortly the fate of 
these two thickenings. I will first describe the structure of the eye in the adult. 
Fig. 12 represents somewhat diagrammatically a section passing through the eye 
transversely to the long axis of the head. Only two divisions of the eye are shown, 
but as these are all alike they will be enough for the purpose. Beginning with the 
external surface of the eye, each division is covered by a thickening of the cuticle 
(C) ; beneath this comes a transparent, refractive, nearly spherical body ( kristal 
Tcegel), somewhat flattened in front, surrounded by seven large cylindrical pigmented 
cells (Z). At a short distance beneath the kristal kegel these cells are marked by 
* For an account of the structure of the eye in the different groups of the Arthropoda, see “ Unter- 
suchungen liber das Arthropoden- Auge,” Dr. FI. Grenacher, ‘ Klinisclie Monatsblatter fur Augenlieil- 
kunde.’ Mai-heft xv. Jahrgang. Rostock, 1877. 
3 u 2 
