94 



Journal of the Mitchell Society 



August 



branchial sac extends further back than the right, due no doubt to 

 complications which arise on the right side as a result of the form- 

 ation of the heart. The lateral sacs later become symmetrical. Le- 

 fevre (loc. cit., p. 476) has likewise described the cloaca as develop- 

 ing from the inner vesicle by ventro-lateral folds. However, he de- 

 scribes the folds as meeting each other along the whole length of the 

 body, thus cutting off a median atrium in the posterior as well as in 

 the anterior region. Lefevre (loc. cit.) states that it is the anterior 

 portion of the cloaca that is first cut off, and that the process proceeds 

 posteriorly. Since this also appears to be the case in my material, and 

 since Lefevre merely studied early stages, it is quite likely that the 

 later development of the posterior ends of the peribranchial sacs was 

 overlooked by him. 



A second feature of development worthy of 

 special comment is the origin of the branch- 

 ial stigmata. These are usually described as 

 arising as evaginations from the branchial sac. Ac- 

 cording to the usual account (e. g., Lefevre^) a 

 thickening of the branchial wall occurs, followed 

 by a slight evagination which brings the branchial 

 wall in contact with the peribranchial wall. These 

 apposed surfaces become intimately fused, the 

 stigma breaking through the area of fusion. 



However, in Ecteinascidia, my preparations 

 clearly shoAv that the stigmata are formed in a 

 slightly different way. The first indication of a 

 stigma is an unmistakable thickening of the peri- 

 branchial wall, as indicated at th. in Fig. 2. A 

 slight evagination of this thickened peribranchial 

 area follows ~(f. //;,.). The basement membrane of 

 the epithelial wall of the inner vesicle then dis- 

 appears opposite the thickening, the peribranchial 

 ,. Fig 2— From a sec- and branchial walls thus fusing. This area of 



tion 01 bud, transverse " 



to long axis of latter, fusiou soou bccomcs perforated by the stigma. The 



The portion of the ^ .; & 



section siiown includes serics of stcps in the formation of the stigmata was 



a part of the wall of ^ ° 



the branchial sac. b.tv., followed in fivc buds, all of about the same stage. 



with the aa.ioinmg pen- ' 'to^' 



funr^fm-me'd";ti'''m'a is "^^^^ stigmata later become fringed with cilia in the 

 f^°fh'. represent*''sta''g''es ^^*^^^^^ manner. They first appear in the middle of 

 mZ'^'mlTenchymeTet}: *^^^ ^^^^^' ^^^^ tendency to arrange themselves in 

 '^ ^^^- vertical rows being evident at the beginning. 



Chapel Hill, N. C. 



^ Geo. Lefevre. Budding in Pcrophora. Joiirn. Morph. 14: 387. 1898. 



