87 



segments form a layer extending over the whole surface of the yolk. The end of the 

 rudiment which was originally turned towards the centre of the germinal membrane 

 becomes enlarged and forms the head of the fish ; the point where the edges of the 

 membrane closed together forms the posterior end. After a short time two spherical 

 masses are seen defined in the head of the embryo : these are the first rudiments of the 

 eyes. The centre of the dorsal rudiment begins to be divided by transverse divisions 

 into segments : these segments are blocks of tissue which will afterwards form the large 

 lateral muscles of the fish. At the tail end a small round cavity appears, which 

 afterwards becomes part of the cavity of the intestine. These various structures are 

 seen in Plate XV, Fig. 6, and Plate XVI, Fig. 1, one of which shows a profile, the other 

 a ventral, view of the embryo at the stage now described. 



At this stage scattered black dots have appeared on the external surface of the dorsal 

 rudiment, extending out over the surface of the yolk some distance from the sides of 

 the latter. These are pigment cells, and are situated in the skin : they are usually 

 called chromatophores. 



What I have called the dorsal rudiment is the solid dorsal portion of the young fish 

 containing the rudiments of the brain and spinal cord, of the backbone below these, 

 and of the lateral muscles at the sides. The yolk sac represents the abdomen of the 

 young fish. The intestine is formed as a simple tube beneath the dorsal rudiment 

 resting on the surface of the yolk. It is at first closed at each end, neither mouth nor 

 anus being yet formed 



After this stage a cavity appears at the anterior end of the embryo between the 

 germinal membrane and the yolk. The segmentation cavity previously described 

 ceased to be visible after the envelopment of the yolk by the germinal membrane, the 

 latter having come into contact with the former. The cavity which now appears has 

 exactly the same position as the segmentation cavity. The embryo grows out 

 posteriorly, forming a tail which is independent of the yolk sac. The black chromato- 

 phores increase in number and extend over the whole surface of the yolk sac, at the 

 same time they become branched, sending out short branches in all directions so as to 

 assume a stellate form. Other chromatophores also appear in addition which are of a 

 yellow colour, appearing darker when seen by reflected light. These changes are seen 

 in Plate XVI, Fig. 2. 



As the tail increases in length a fold of the skin is formed in the middle line along 

 its dorsal and ventral edge ; the dorsal fold extending to the head of the embryo. The 

 other changes which have taken place by the time the young fish is hatched will be 

 understood by reference to Plate XVI, Fig. 3, which shows the fish immediately after 

 hatching. The yolk is now considerably diminished and the intestinal tube opens 

 behind it by the anus. The tail measured from the anus is as long as the distance 

 from the anus to the front of the head. The wide membranous fold is seen extending 

 along the middle ventral line of the tail and along the median dorsal line of the fish to 

 the back of the head : this is the primordial median fin. There is still no mouth. 



