vesicle Is always derived from the ectoderm of the parent, 

 and gives rise to the ectodermal covering of the bud. The 

 origin of the inner vesicle, however, is not the sane in all 

 Ascidians. In the Botryllid ae it arises, in both larva and 

 bud, from the perlbranchial wall, which is formed in the first 

 place from, the ectoderm of the embryo. This vesicle is, 

 therefore, ultimately of ectodermal ori^^in in Botryll us , 

 whereas in al] other Ascidians it comes from an endcdermial 

 structure of the parent. 



Although derived in the two cases from different 

 germ-layers, the inner ve'sicle may go to form the same organs 

 in the bud, but organs which are of widely different origin 

 in the larva; for example, the digestive tract and nervous 

 system. (Pljort, Botryl] us (8), Distaplia (10), Glcssophorum 

 (9). 



The fate of the inner vesicle is consequently di- 

 rectly opposed to the application of the germ-layer theory in 

 its ordinary meaning to the Ascidian bud. 



In Perophora viridis the important part played by 

 cells, which there is every reason to regard as mesodermal 

 cells, in the bud-development, only increases the disagree- 

 ment. Organs which in other Ascidians are formed from ec- 

 toderm or endoderm, are here of mesodermal origin. The ner- 

 vous-system and pericardium, v/hich are respectively ectodermal 

 and endodermal productions in the larva, develop out of free 

 mesodermal cells. 



Since in the development of the bud and larva the 



