854 ARTHUR WILLEY. 



body-cavity, and contains loose mesoderm- cells derived from 

 the two lateral mesodermic bands. 



(5) The position of the prseoral lobe (fixing stolon), which, as 

 the name indicates, is at first quite anterior, becomes reversed 

 by the rotation of the body of the Ascidian through an angle 

 of 90°. 



(6) In Clavelina it can be observed that the atrial involu- 

 tions occur at the hinder extremity of a longitudinal groove 

 on each side, the groove being of a very appreciable length, and 

 gradually flattening out from behind forwards. 



(7) The walls of the atrial cavities of Ascidians are essentially 

 ectodermic, there being no diflfereuce in this respect between 

 the somatic and visceral walls. 



(8) The pyloric gland of Ascidians arises as a simple hollow 

 csecal diverticulum from the stomach at the point of junction 

 between stomach and intestine, and is at first lined by a 

 columnar or cubical epithelium. 



(9) The pericardium of Ascidians (in confirmation of van 

 Beneden and Julin) arises from the endoderm of the branchial 

 sac, and the heart has no endothelium. 



(10) The heart of Ciona arises by the splitting apart of the 

 two layers of the septum which primarily divided the peri- 

 cardium into two halves. 



(11) The prseoral lobe (fixing stolon) serves for the common 

 insertion of all the longitudinal muscles of Ciona. 



(12) The development of Clavelina as a whole, in comparison 

 with that of Ciona, is greatly modified in the direction of 

 abbreviation. This is to be correlated with the facts that the 

 embryonic development of Clavelina takes place in the peri- 

 branchial cavity, and that the eggs contain a great deal more 

 yolk than those of Ciona. 



