PYROSOMA THE FOUR PRIMARY ASCIDIOZOO1DS. 



411 



layers, the external layer (Fig. 203 A, el) being the rudiment of the 

 elaeoblast, and the inner that of the so-called pericardial strands, 

 which must not be confounded with the pericardial tube mentioned 

 above, an organ that disappears at an early stage (Fig. 203 A, pc and 

 />(,'). The cells of the elaeoblast-rudiment soon increase in size and 

 form a rather high cylindrical 

 epithelium. At a later stage, 

 they are less regular in their 

 arrangement, vacuoles develop 

 within them and they change 

 into large elements, resem- 

 bling vegetable parenchyma- 

 cells, and thus assume the 

 features characteristic of the 

 elaeoblast-tissue (Fig. 203 6"). 

 The elaeoblast is here at first 

 paired and consists of rounded 

 groups of cells lying in the 

 posterior part of the body 

 which cause the surface of 

 the body to bulge out some- 

 what (Figs. 194 and 200 el). 

 The inner layer of the 

 paired mesodermal rudiment 

 just described gives rise to 

 two cell-strands which develop 

 differently (Fig. 203, pc, pc'). 

 The strand to the right 

 extends somewhat further 

 forward than the other, and 

 its anterior end becomes 



FIG. 203. Transverse sections through the 

 distal region of an Ascidiozooid of Pyrosoma 

 in three consecutive stages of development 

 (after SALENSKY). ec, ectoderm ; el, rudi- 

 ment of elaeoblast ; en, entoderm; es, paired 

 endostyle-folds ; g, genital strand ; pc, right 

 pericardial strand, or pericardial vesicle ; 

 pc', left pericardial strand. 



transformed into a closed 



vesicle, the pericardial vesicle 



(Fig. 203 B and C, pc). The 



heart develops through the 



thickening of that wall of the 



vesicle which is in contact 



with the alimentary canal and its invagination into the vesicle. 



The heart consequently forms here in the same way as in all other 



Tunicates. 



The right pericardial strand (Fig. 203, pc) is not completely used 



