352 ARTHUR WILLEY. 



of such an organ in Ciona, although it was held as probable by 

 these authors that it occurred in all Tuuicates (7, p. 300). 



Figs. 25 and 27 show the first origin of the pericardium in 

 Clavelina lepadiformis. From the ventral wall of the 

 pharynx in the pericardial region a single layer of cells is 

 produced by some sort of delamination^ but whether through 

 special cell divisions or through the separating out of individual 

 cells I could not exactly determine, but incline to regard the 

 latter as the method. Its endodermic origin is, therefore, 

 undoubted, since it arises at a stage before the ectoderm has 

 sprung away from the endoderm to give rise to the body-cavity 

 between these two layers. The mesoderm, therefore, which, 

 as shown in fig. 25, lies dorsally on each side of the nerve- 

 tube, can have no part in the origin of the pericardium. 



In Ciona the first appearance of the pericardium presents an 

 essentially similar aspect to that described for Clavelina, but 

 on a much smaller scale, and at a later period of the develop- 

 ment, when a body-cavity is already present. As for the actual 

 double nature of the pericardial '' ebauche" at the very first, I 

 am not able to confirm van Beneden and Julin, as my figures 

 will show; but I may point out that their figure (7, pi. ix, 

 fig. 1) representing the earliest stage of the pericardium is 

 drawn from an optical section. For the further development 

 of the pericardium and heart in Clavelina I have nothing 

 essential to add to the account given by van Beneden and 

 Julin. 



The first sign of the pericardium in Clavelina occurs at a 

 very early stage of development, before any trace of the atrial 

 involutions is present, and before the appearance of pigment 

 in the cerebral vesicle ; and the statement of Seeliger (31) 

 that it arises at a comparatively late period, after the formation 

 of the atrial involutions and the appearance of pigment in the 

 brain as a hollow evagination from the ventral wall of the 

 branchial sac, is quite erroneous. Later on, the at first solid 

 " ebauche'^ of the pericardium acquires a double lumen, but 

 the septum between the two lumiua is apparently never com- 

 plete, and soon disappears, and the heart then arises as an 



