lus, that the bud-rudiment consists of two vesicles, one with- 

 in the other, the outer being derived from the ectoderm, and 

 the inner fron the peribranchial wall of the parent. He al- 

 so observed the splitting up of the inner vesicle to form the 

 median branchial sac and the lateral peribranchial spaces, 

 but neither Metschnikoff nor Krohn (15) who confirmed these 

 observations on the Botryllus bud, sav/ at all clearly the de- 

 tails of the development. This was left for Kov/alewsky, who 

 described many of the internal processes occurring in the 

 bud-development of Perophora Lister i . He show^ed that in Pe - 

 rophora also the young bud is composed of two vesicles, the 

 outer one being derived from tr.e ectodem and the inner from 

 the partition wall of the stolon. According to Kowalewsky's 

 account the endodermal or inner vesicle becomes divided com- 

 pletely into three portions, the two lateral fusing dorsally 

 and form.ing the peribranchial cavity, and the median giving 

 rise to the branchial sac. I shall try to show below that 

 in Pero phora viridls , at all events, the peribranchial cavity 

 is formed by quite a different process. 



The origin of the bud as a double-v.'alled vesicle 

 has been verified by all subsequent investigators, and tho- 

 roughly established as a type of development cormion to all 

 Ascidians whose budding has been studied. 



The outer wall of tne vesicle is directly derived 

 from the ectodem of the parent animal, and becomes the ecto- 

 dermal covering of the bud. According to the majority of 

 investigators this outer layer takes no active part in tiie 

 furtner development, but Salen,3ky (27) and Oka (20), as v;ill 



