OF THE PARASITIC ISOPODA. 
507 
Fig. 3 shows the ventral side of the egg at this stage. The segments of the head 
and thorax are quite distinct ; hut in the abdominal region they have not yet become 
distinguishable. A depression (M) at the anterior end of the embryo is the involution 
to form the fore-gut ; it is very slight, and appears as a dark mark when seen from 
the surface. Extending from the mouth towards the hinder end of the body in the 
median line, is a dark mark caused by the surface of this portion of the embryo being 
lower than the rest. It is along this line that the nervous system will be developed. 
The ventral half of the egg is covered by a layer of flattened cells extending in all 
directions from the edges of the Keimstreif, and in the hardened egg appears to the 
naked eye much lighter in colour than the dorsal surface. With the microscope it 
may be seen that there are cells also on the dorsal surface, but they are very much 
thinner and less easily seen than those on the ventral surface. On the dorsal surface 
of the egg at the anterior end is a transversely elongated band of cells (fig. 4, D) 
connected with tire embryo only by means of the flattened dorsal cells. This is 
the rudiment of a, peculiar organ which at a latter period occupies the dorsal part of 
the first thoracic segment, and which I shall speak of in future as the dorsal organ. 
It may be well at this point to say a few words about this organ, and the various 
structures which have been regarded as identical with it in other Crustacea. In 
Cymothoa it does not long retain the structure described above, but soon becomes 
circular, and the cells composing it more columnar. As development proceeds it 
increases in size, and causes a considerable depression in the yolk ; at the same time, a 
cavity is developed in its interior, of the shape shown in the diagram A (fig. I, 
ivoodcut). It is now seen to be in connexion with the inner egg-membrane, though 
Diagrams showing dorsal organ. A, Cymothoa; B, Asellus. (Van Beneden, Bull. Acad. 
Roy. Belg., t. xxviii.) 
whether this has been the case all along I am unable to say. Having attained 
this form, it undergoes no further modification during the period in which I have 
been able to observe it. Considerably after the embryo had escaped from the egg- 
