BUDDING IN TUNIC AT A. 57 



tiation of the ventral wall of the anterior part of the dorsal 

 tube ; that is, that the brain is of endodermal origin, — if our 

 interpretation of the layers of the Botryllus bud is correct. 

 Pizon, on the other hand, is equally sure that the brain and 

 dorsal nerve cord have an origin altogether distinct from the 

 dorsal endodermal tube. According to him the nerve tube 

 is recognisable as a slender thread of considerable length in 

 the stages in which the dorsal tube has acquired its anterior 

 opening into the pharynx, and has not yet lost its primitive 

 posterior communication, and in which the saddle-shaped 

 cloacal and peribranchial vesicle is separated from the 

 pharyngeal vesicle. Anteriorly the nerve-cord lies close 

 beneath the dorsal tube ; but posteriorly the two are 

 separated from each other by the cloacal sac. The dorsal 

 tube terminates almost immediately, but the nerve-cord is 

 continued for some little distance backwards between the 

 gut and the floor of the cloaca, until on account of its great 

 delicacy it is no longer recognisable in transverse sections. 

 If Pizon's observations are correct, the nerve-cord is distinct 

 both from the dorsal tube and from all the other organs and 

 layers of the bud from the first, and its juxtaposition to the 

 dorsal tube is to be regarded as of quite secondary import- 

 ance. Unable to trace any intimate connection between the 

 nerve-cord in its earliest stages and the other organs of the 

 bud, Pizon inferred that the cord is not formed in situ, but 

 as a special prolongation from one of the nerves of the parent 

 — an inference which he drew from his observation that the 

 cord in early stages is prolonged backwards beyond the 

 dorsal tube, becoming finer and finer in its course. Possibly 

 in still earlier stages this fine portion of the cord is directly 

 prolonged through the stalk of the bud into the body of the 

 parent ; but, as we have seen, the evidence presented by 

 Pizon upon this point is inconclusive. There is still another 

 account of the development of the brain in the buds of 

 Botryllus. Oka has also recognised the existence of the 

 dorsal tube in the buds of this form, although he overlooked 

 the earliest stages in its development. He noticed, too, the 

 longitudinal tract of proliferating ectoderm cells. These 

 were regarded by Pizon as a band of incipient mesenchyme 



