No. 2.] DEVELOPMEATT OF PETROMVZON: 29I 



eigenen Ventilationsapparates des Geruchsorganes ausser dem 

 Athcmorgane. Diescn Zweck hat dcr Spritzsack dcr Pctromy- 

 zon und der segelartigc Ventilator am Gaumen der Myxinoi- 

 den." 



The only convenient place for the unobstructed develop- 

 ment of the naso-palatal canal is the median position it occu- 

 pies, as elsewhere the great muscles of the suctorial disc and 

 tongue would impede its action. These considerations would 

 seem to explain the coalescence of the originally paired olfac- 

 tory organs, and the transferrence of the opening of the nasal 

 pit from the ventral to the median dorsal line of the head. 



Dohrn (12) has propounded very similar views to those just 

 given, with regard to the development of the upper lip and its 

 effect upon surrounding organs. But he does not seem to have 

 read my preliminary papers (32 and 33) in the " Zoologischer 

 Anzeiger " and the " Quarterly Journal of Microscopical Science," 

 in which these considerations were all outlined. 



3. The Eye. — Wilhelm MUUer (6) was the first to de- 

 monstrate that the eye of Pctromyzojz is developed in essentially 

 the same manner as in the higher vertebrates. His observations 

 leave little to be done, except with regard to the earlier stages. 

 The first rudiment of the eye appears about the sixteenth day 

 after impregnation, in the formation of the optic vesicles, which 

 are slender hollow stalks of strikingly small size, the walls of 

 which consist of a single layer of columnar epithelial cells. 

 The vesicles are covered by a delicate layer of more or less 

 spindle-shaped mesobldstic cells, a continuation of those which 

 envelop the brain. About the eighteenth day (Fig. 24, PI. X) 

 the retina is formed by the elongation of some of the cells in 

 the anterior wall of the primary optic vesicle, v/hile the pos- 

 terior wall becomes thinner and its cells flatter ; the two v/alls 

 are now in close contact, so that in the region of the retina the 

 cavity of the primary vesicle is obliterated, though it still persists 

 in the stalk. The great peculiarity in this mode of development 

 consists in the very small part of the vesicle which becomes the 

 retina, as compared with the retinal region in other vertebrates. 

 This is no doubt for the most part due to the retardation which 

 affects the formation of the eye, for during larval life this organ 

 is entirely functionless ; but it is also probably due in part to 

 the fact that in the higher vertebrates the brain and organs of 



