58 SCIENCE PROGRESS. 



cells ; but Oka states that the cells produced along this line 

 of proliferation migrate inwards on each side of the dorsal 

 tube, and reunite on its ventral side to form the brain and 

 nervous system. 



There are thus three distinct views as to the origin of 

 the nervous system in the buds of Botryllus. According 

 to one the brain is produced from the dorsal (endodermal) 

 tube ; according to another from a proliferating streak of 

 ectoderm ; while, according to the third, the nervous system 

 is not produced from either of the constituent layers of the 

 bud, but by means of a special rudiment derived from the 

 parent organism. The first view is manifestly improbable, 

 although based on an apparently careful piece of research ; 

 the second view finds a parallel in the observations of Van 

 Beneden and Julin on the buds of Clavelina, and is quite 

 consistent with the general principles of embryology ; the 

 third view is fairly well founded, and equally harmonious 

 with the germ-layer theory. Research alone can settle a 

 point so much disputed; but we may mention that although 

 at first sight the second of these views may commend itself 

 most readily upon a priori grounds, the facts upon which 

 this view is based were fully known to Pizon, who entertained 

 the same opinion for a time, and rejected it after renewed 

 investigation. The balance of probability accordingly rests 

 with Pizon's conclusions ; and it seems to me that the 

 recent research of Salensky (27) upon the development of 

 the twin-embryo of Diplosoma supplies a powerful argument 

 from analogy in support of Pizon's special theory. In this 

 remarkable embryo the primary individual early gives rise 

 (by a process which is intermediate between fission and 

 budding) to a second individual resembling itself; and the 

 nervous system of the second or ventral individual is formed 

 trom a special nerve-tube which grows out laterally from 

 the neural tube of the primary individual. Whatever 

 opinion may be held as to the relations between fission 

 and budding in Tunicata the case of Diplosoma furnishes 

 a basis of fact which renders Pizon's view of the origin of 

 the nervous system in the Botryllus bud worthy of full 

 consideration. It may be that future investigators will 



