DEVELOPMENT OF THE HEAD IN GOBIUS CAPITO. 347 



No head cavities seem ever to be present in Gobius, though in 

 Syngnathus a premandibular cavity occurs. Fig. 9 shows the 

 premandibular mesoderm fairly defined on the right side 

 {Pmd. mes.). In this section it passes continuously across the 

 middle line below the brain, to join the corresponding mass on 

 the opposite side. This is the bridge of mesoderm, so well 

 known from Selachian embryology, which lies behind the in- 

 fundibulum ; at its median side and in front the premandibular 

 mesoderm fuses with the mesoderm surrounding the eye. One 

 point of fusion is shown on the left side in Fig. 9. A similar 

 phenomenon is described by Kupffer (13) for Petromyzon. This 

 author considers the cells lying round about the eye to be 

 chains of nerve-cells. 



Behind the eye, layers in the mesoderm cannot be dis- 

 tinguished (fig. 10) at this stage. 



The mesoderm surrounding the eye and corresponding meso- 

 derm in other regions has been made the subject of special 

 investigation by Goronowitsch (5). As far as my researches 

 go they confirm his statements. This mesoderm is derived, as 

 it seems, from the " Ganglienleiste," or " neural ridge" of 

 Marshall, whose role, according to Goronowitsch, has been 

 entirely mistaken by many embryologists. At the same time 

 the fusions which occur between this mesoderm and the 

 premandibular mesoderm make it difficult, at least in Teleostei, 

 to decide whether " sclerotomic " elements derived from the 

 premandibular mesoderm may or may not take part in the 

 formation of the skeletal structures which arise from this tissue 

 much later. 



The position of the premandibular mesoderm is shown in the 

 diagram fig. 2, which also shows the extent of the coelom, 

 which, as is well known, forms the pericardial cavity. The 

 coelom actually borders on the mouth involution — that is, it 

 extends almost to below the eye. The somatopleure {So. pi.) is 

 thin, but the splanchnopleure forming the inner pericardial wall 

 is thickened. On the third following day the premandibular 

 mesoderm is taking its position round the eye so as to form the 

 eye muscles, which later are supplied by the oculomotorius. 



