DEVELOPMENT OF THE HEAD IN GOBIUS CAPITO. 339 



On the third day important changes have taken place. 

 There is a considerable cranial flexurCj inasmuch as the optic 

 stalks are distinctly ventral. The eye has rotated correspond- 

 ingly, and a choroid fissure has been formed. The nose is 

 turned downwards and forwards. The pineal organ is a small 

 solid knob. The cavity of the fore-brain is formed, and, as 

 Goronowitsch states for the salmon, is a dorsal outgrowth from 

 the cavity. The mid-brain has become very broad, and 

 possesses a large cavity, which, however, is being invaded by 

 the tori longitudinales, which appear as thickenings of the 

 dorso-lateral wall. The cavity of the medulla oblongata 

 shows the well-known beaded constrictions. The infundi- 

 bulum is large, and its cavity is narrow but deep. As yet there 

 is no indication of lobi inferiores or of the formation of the 

 saccus vasculosus. Behind the infundibulum the isthmus and 

 base of the cerebellum are raised from the yolk, leaving a space 

 occupied by loose mesodermic tissue. The wall of the brain 

 from the pineal organ to the infundibulum — that is, the region 

 of the '^vordere Endnaht " of His — is epithelial in the median 

 line. 



At this stage nerve-fibres are first dififerentiated at the peri- 

 phery of the brain. 



Fig. 4 represents a model of the brain at the fifth day. The 

 model has been cut in halves at the middle line in order to 

 show the relation of the cavities. Viewed externally it shows 

 practically the same characters as are depicted by Goronowitsch 

 for the salmon embryo of the thirtieth day. 



Posteriorly, to the left in the figure, the medulla oblongata 

 is cut in section. The conditions of the first day have been com- 

 pletely reversed, in that the brain in this region is broader 

 than deep. The fourth ventricle is roofed in by thin membrane. 

 The roofing membrane passes into the cerebellum, which at 

 this stage passes near the middle line insensibly into the mid- 

 brain. The cavity of the mid-brain is now mainly filled by the 

 massive torus longitudinalis. In the figure the commissures 

 present at this stage are represented in red. Where the mid- 

 brain passes into the fore-brain the posterior commissure occurs. 



