_19_ 



also in Dipnoi, the neural hypophysis becomes closely and almost 

 inseparably connected with the oral hypophysis. Usually the "An- 

 lage" of the oral hypophysis lies in the region of the mouth epiblast, 

 in Petromyzon and Myxine it lies in front of, and outside the mouth. 

 The process by which it got into the mouth involution cannot be 

 explained without numerous figures. 



According to Dokrn, in Petromyzon the oral hypophysis arises 

 as an invagination of epiblast in front of the mouth, and between oral 

 and nasal depressions. This pit grows towards the base of the in- 

 fundibulum, and comes into close relationship with the end of the 

 notochord i. e. with a structure derived from hypoblast. This direction 

 of its growth is also towards a special process of hypoblast, with 

 which however in Petromyzon it does not fuse. In Myxine, although 

 unfortunately the development is not yet known, we may assume that 

 it does fuse with a corresponding process of hypoblast, tor in this 

 animal it opens throughout life into the gut. 



In Ammocoetes it gives oÖ" a certain number of gland-folhcles, 

 which according to Dohrn become pinched off from the oral hypo- 

 physis in the Petromyzon. Of the certainty of the latter point I am 

 not yet quite convinced. In Myxine I find numerous small glandular 

 follicles opening into the oral hypophysis. In Petromyzon and Myxine 

 the neural hypophysis is present, and as I believe, not rudimentary. 

 It appears to supply nerve fibres to the oral hypophysis. 



In Hippocampus Dohen finds traces of a paired origin of the 

 oral hypophysis. This is important. 



The oral or glandular hypophysis I propose to divide into two 

 parts , the duct I may call main oral hypophysis and the glandular 

 part I may call glandular hypophysis. 



The whole hypophysis is without doubt in nearly all cases rudi- 

 mentary and of little or no functional importance. 



Which of the views held till now was the most likely, it is quite 

 impossible to say, for unfortunately none of them explained the whole 

 complex, either the neural hypophysis, or the oral hypophysis was 

 left obscure in every case. But recently we have ol)tained a great 

 increase of knowledge in the developmental history of Annelida, thanks 

 above all to Kleinenberg and Salensky. 



From the results of Kleinenbeeg's work more especially we are 

 placed in a position to compare the structure and development of 

 the hypophysis with those of certain organs in worms. 



To my mind the comparison which follows is one of the neatest 

 in the whole range of comparative morphology, but before proceeding 



2* 



