56 SCIENCE PROGRESS. 



form, about which is being waged at present a controversy 

 which must be crucial in its results. The question turns 

 upon the relation between the neural tube and the so-called 

 hypophysis or subneural gland. The existence of this 

 organ, which lies beneath the brain in most Ascidians, has 

 long been known ; but its significance has been the subject 

 of much dispute. It has, however, usually been regarded 

 as a diverticulum of the anterior part of the pharynx, 

 homologous with the hypophysis of Vertebrata. Pizon has 

 shown that in the larvae as well as in the buds of Botryllus 

 this organ arises as a forwardly directed tubular diverticulum 

 from the dorsal side of the hinder part of the primitive 

 endodermal vesicle ; and that after effecting a secondary 

 union with the wall of the pharynx anteriorly, it loses its 

 primary communication with the endoderm, and gradually 

 atrophies from behind forward until only its anterior portion 

 is left. This persisting portion becomes the subneural gland 

 and ciliated vesicle of the adult Ascidian. To the primary 

 diverticulum, in its whole extent, Pizon has given the descrip- 

 tive name "dorsal tube". In the bud, Pizon shows that the 

 cloaca and peribranchial sacs are not formed independently 

 of one another, but are constricted off from the dorsal side 

 of the primitive endodermal vesicle as a single saddle-shaped 

 vesicle. It follows from the posterior situation of the 

 diverticulum, which gives rise to the dorsal tube, that the 

 latter also becomes removed from the pharyngeal vesicle 

 pari passu with the separation of the cloacal vesicle. In this 

 way the dorsal tube presents itself in later stages of develop- 

 ment as a diverticulum of the anterior wall of the cloaca. 

 Pizon's observations upon all these points in the budding of 

 Botryllus were fully confirmed by the independent researches 

 of Hjort, whose general conclusions, however, are marred 

 by his assumption of the ectodermal origin of the inner (or 

 endodermal) vesicle. Hjort's attitude with regard to this 

 point has already been discussed. When we pass from the 

 development of the dorsal tube to consider the origin of the 

 nervous system itself, we discover serious differences between 

 the accounts of the two investigators. Hjort maintains that 

 the brain of the Botryllus bud is produced by local differen- 



