DEVELOPMENT OF THE HEAD IN GOBIUS CAPITO. 345 



lumeu with the exterior. The dotted yellow line (which 

 towards its anterior limit must be regarded as lying below the 

 blue line) represents the upward prolongation of endoderm, 

 which fuses with the mesoderm surrounding the eye (fig. 2,*). 



Figs. 3 and 11 show the condition two days later. Fig. 11 

 represents a sagittal section passing through the eye, the lens 

 at its anterior border — the mid-brain and ear. The eyes and 

 brain are now separated from the yolk by various organs. 

 The mouth and hypophysis ingrowths are still solid, but at 

 this stage they are no longer two separate involutions. The 

 posterior portion is surrounded by condensing mesoderm, which 

 forms upper and lower jaws, and is separated from the yolk not 

 only by the forward-growing lower jaw, but also by the 

 pericardium and its cavity, which are in process of extension 

 forwards. From the eye the united ingrowths are separated by 

 the mesoderm of the upper jaw,'and by the suborbital ganglion 

 of the lateral line. The ectoderm is specially thickened at the 

 angle where it passes over the yolk, beneath the choroid 

 fissure in fig. 11. 



In the same section are seen the mandibular artery [Bl. v.) 

 behind the lower jaw, the rudimentary hyomandibular cleft 

 {H. M.), and the hyoid cleft [Hy.). 



Following the same series of sagittal sections towards the 

 middle line, the ectoderm is found to be perfectly continuous 

 to the middle line. Beneath the infundibulum two layers of 

 cells may be distinguished, the upper consisting of cubical 

 cells closely packed together, and the lower of much flattened 

 cells, which form a flat epithelium next to the yolk. This 

 lower layer was the only one present in the first described 

 stage. The cells of the layer bordering on the infundibulum 

 can but be derived from the anterior thickened ingrowth of the 

 previous stage. They are the cells destined to form the 

 hypophysis. 



Transverse sections put the conclusions derived from this 

 series of sagittal sections beyond doubt. 



The ectoderm of the angle of the mouth has fused indis- 

 tinguishably with the endoderm, which also now gives rise to a 



