Embryology of the River-Lamprey . 37 



ducts, and finally in the lateral plates. It is asserted by 

 A. Shipley (fig. 11) that the Wolffian ducts or the segmental 

 organ develop before the protovertebrge. I have seen no 

 evidence of this in my preparations. The figure in question 

 is altogether too diagrammatic. 



The pro to vertebral segments are not cubical, but are drawn 

 out into a point at the lower inner angle towards the notochord. 



On the Origin of the Sense-Organs. 



I have not observed the formation of the auditory apparatus 

 in embryos less than 160 hours old. There appears at the 

 side of the brain a pit-shaped invagination of the ectoderm 

 which is equally distinct in transverse as well as in longitu- 

 dinal sections. The cells which are situated in the centre of 

 the pit are somewhat longer than the remainder. After some 

 time the pit closes and becomes a capsule, the future labyrinth 

 of the auditory apparatus. At this period all cells, both of 

 the skin as well as of the nervous system, are still pretty 

 richly packed with yolk-lamellge. The intermediate space 

 between the auditory capsule and the brain is very incon- 

 siderable, and subsequently almost completely disappears. 



The formation of the eye takes place during the two 

 hundredth to the two hundred and fortieth hour after fertiliza- 

 tion. At the side of the upper surface of the fore brain, almost 

 immediately behind the olfactory pit, we find a sac-shaped 

 expansion of the cerebral wall. It extends backwards and 

 downwards, and can be better seen in longitudinal sections, 

 though in order to obtain a complete representation of it both 

 classes of sections must be examined. At first the cavity of 

 the saccule is exceedingly narrow, but it subsequently becomes 

 somewhat wider, especially at its outer end. 



The earliest rudiment of the paired eyes in the lamprey 

 presents more similarity to the rudiment of the third eye in 

 many animals than to that of the eyes of the Vertebrata in 

 general. For in this case we find a relatively long tubular 

 stalk, the outer and somewhat expanded end of which is only 

 utilized for the formation of the retina. The outer wall of 

 the eye-stalk grows thicker and becomes indented like a pit, 

 just as the retina is generally developed. The epithelium of 

 the skin, which lies opposite the rudiment of the eye, takes no 

 part in its formation. During this time no trace of the lens 

 can be discovered. 



The formation of the nasal pit proceeds by a pit-shaped 

 invagination of the outer layer, as has already been frequently 

 described. 



