432 TUNICATA. 



owes its origin to the mesoderm, i.e., to the lamella lying on the 

 right side of the body, which is continued back over the posterior end 

 of the wall of the branchial cavity, and there forms a vesicle (Fig. 

 2 16, p) in which we recognise the first rudiment of the pericardia] 

 vesicle.* A swelling of the thickened dorsal wall of the vesicle, 

 which projects into its lumen, and which, though at first solid 

 becomes hollow later, is the rudiment of the heart proper; this con- 

 sequently develops in just the same way as in the Ascidiacea (p. 370). 

 The blood-vessels apparently arise as spaces within the gelatinous 

 connective-tissue which, in later stages, fills the primary body-cavity. 

 It should be mentioned that, in the Salpidae, as Todaro has pointed 

 out and figured, the blood-vessels seem to be lined throughout with 

 a cellular intima (Fig. 207 B, l>). In this respect this group 

 would seem to differ from the Ascidiacea, in which, according to 

 van Beneden and Julin, such an intima is wanting (rf. pp. 363 

 and 371). 



The elaeoblast (Fig. 216, eb), the rudiment of which has- already 

 been described (p. 425), attains its full development only in the later 

 stages of embryonic life, and, after the birth of the embryo, under- 

 goes gradual degeneration [by phagocytosis, according to Korotneff 

 (No. XIX.)]. It is a mass of large polygonal cells, which are filled 

 with reserve nutrition. The remarkable resemblance between the 

 elaeoblast and the degenerating larval tail of Doliolum (p. 388) caused 

 Salensky to assume that this problematical organ is the homologue 

 of the tail and the chorda of the Ascidian larva. But the presence 

 of the rudiment of the elaeoblast, as we shall see in the buds of the 

 Salpidae and of Pyrosoma, is not very favourable to this view. 

 Physiologically, the elaeoblast is probably, as Leuckart suggests, 

 a reservoir of food-material, which is gradually used up as the 

 embryo develops. 



At a later stage of development the rudiment of the stolon can 

 be seen (Fig. 216, at). This consists first of a diverticulum of the 

 pharyngeal wall lying at the posterior end of the endostyle, and is 

 turned toward the left side of the body. The ectoderm soon bulges 

 over this entodermal diverticulum. The space between the two 

 layers is, according to Seeliger (No. 105), filled with mesenchyme- 

 cells, the short conical stolon thus consisting of three germ-layers 

 (p. 495). 



* [According to Korotneff (No. XVIII.), the pericardium arises as in other 

 Tunicates as a diverticulum <>f bhe pharynx. — Ed.] 



