388 TUNICATA. 



vesicle.* It should here be mentioned that the dorsal closure of the 

 cardiac tube is brought about by a histologically differentiated lamella 

 (the " mittelfeld" of Grobben) as to the development of which, how- 

 ever, we have no detailed information, but we are reminded of the 

 participation of the epicardial lamella in the formation of the heart 

 in the Ascidians (van Beneden and Julin, p. 370). 



The muscle-hoops develop in the way described by Leuckart for 

 8al}xi democratica (see p. 431), through the fenestration of the meso- 

 dermal lamella, these perforations separating one muscle-hoop from 

 another. 



In this way, the general course of the most important systems of 

 organs occurring in the first " nurse " generation is indicated (Figs. 

 180, 243, 245, p. 475). To these, two stolons connected with the 

 formation of buds have to be added. One of these (Figs. 180, /•, and 

 245 A, rs) lies behind the fifth muscle-hoop, close to the posterior 

 end of the pericardial vesicle ; this we shall call the ventral stolon 

 (the rosette-shaped organ). The second ox, dorsal stolon (Figs. 180, d, 

 245, ds)-\ arises from the dorsal surface in the seventh intermuscular 

 space and forms a geniculate process pointed posteriorly, into the 

 base of which an open coil of the seventh muscle-hoop extends (Fig. 

 243, st'). We shall have to enter into the details of the structure 

 and development of these stolons and of their relation to the forma- 

 tion of the subsequent generations later (p. 470). 



After the young barrel-shaped "nurse" has developed fully, the 

 provisional larval organs gradually atrophy. While the internal 

 parts undergo fatty degeneration and the cells become mixed with 

 the blood, the ectodermal envelope gradually shortens so that the 

 ectodermal vesicle and the larval tail soon form merely a rounded 

 prominence on the body of the " nurse ". This outgrowth strikingly 

 resembles an embryonic organ consisting of reserve nutrition found 

 in the Thaliacea, the so-called elaeoblast, a fact which makes the 

 derivation of the latter from the transformed tail of the Ascidian 

 larva, attempted by Salensky, appear somewhat probable (see below 

 p. 432) 



The first "nurse" generation of Doliolvm, at a later period, as 

 Fol first pointed out, undergoes a remarkable metamorphosis, the 



* Grobben's statements as to the formation of the heart in the larval 

 Doliolum have beem misunderstood and misrepresented by Uljanin. 



+ [Th is is better termed the dorsal outgrowth, as it does not itself give rise 

 to buds, but receives those structures in rudiment from the ventral stolon and 

 miiK gives attachment to them (pp. 472-47G). — Ed.] 



