the posterior floor of the pharynx just to the right of the 

 median line. It does not lie horizontal, i.e. parallel with 

 the surface of the stolon, but the posterior is higher than 

 the anterior end. This inclination is seen in Figs. 4 and 

 5, in v/hich the pericardii;ra is indicated at pc . In the 

 latter figure, which shows about its final position, the peri- 

 cardium is seen to extend from a point at a level nearly as 

 high as the upper end of the stomach straight down to the 

 stalk which connects the bud with the stolon. 



THE DORSAL TUBE AND GANGLION. 



Of all the organs of the Ascidian bud, that which 

 has given rise to the greatest amount of discussion is the 

 nervous system. Its origin and development have been mat- 

 ters of much dispute, and so wide is the difference of opin- 

 ion concerning points of fundamental importance, that there 

 is little hope at present of harmonising the conflicting 

 statements of the various authors. 



A close relation between the dorsal tube and gan- 

 glion has been affirmed by many v/ho hold to a common origin 

 of the two, but is strenuously denied by others who assert 

 that the dorsal tube arises independently, and has nothing 

 whatever to do with the nervous system. Different authors 

 have ascribed to these striictures an ectodermal, a mesodermal 

 and an endodermal origin, and have thereby exhausted the en- 

 tire series of possibilities. 



•40- 



