360 ARTHUR WILLED. 



relations of the atrial cavity to the stigmatic region of the branchial sac. 

 Zeiss, 2, D. 



Pig. 29. — Transverse section through a larva of Clavelina older than the 

 preceding. The neuro-hypophysial canal lies closely applied against the left 

 Vfall (right of the section) of the cerebral vesicle. The two atrial cavities are 

 shown lying against the folds into which the stigmatic region of the branchial 

 sac is thrown. The endostyle here and in preceding figure is also cut trans- 

 versely. Zeiss, 2, D. 



Fig. 30. — Optical section, showing mode of formation of the heart in Ciona 

 by a splitting apart of the layers of the pericardial septum. Zeiss, 4, D. 



