ASCIDIACEA — FORMATION OK ORGANS IN THE HUD. 463 



In the Diplosomidae the process of budding and bhe formation of double 

 individuals begins even in bhe free-swimming larva, but in the Didemnidae 



this is not the case. 



But for the statements made 1>\ JOURDAIN we might be tempted to derive 

 the remarkable budding processes in these two families from a primitive 

 longitudinal fission of the parent. 



E. Development of the Organs in Asexually Produced 



Individuals. 



The development of the inner parts of the bud has been described 

 by Metschnikoff (No. 41), Ganin (No. 55), Kowalevsky (Nos. 60 

 and 61), Giard (Xo. 57), Della Valle (No. 68), Seeliger (Nos. 

 66 and XXXIV.), van Beneden and Julin (Nos. 10 and XVII.), 

 H.iort ( Nos. 59 and XVI. ), Oka (No. 64«), and in the still more recent 

 works of Caulery (Xo. IV.), Lefrvre (Xo. XXII.), and Ritter 

 (No. XXVIII.). In our account of these processes we have followed 

 Kowalevsky, whose careful researches have been confirmed with re- 

 gard to the development of the nervous system and the pericardial 

 vesicle by van Beneden and Julin. 



The bud is at first a hollow body consisting of two or three layers 

 (Fig. 229, kn). The outer layer is the ectoderm (ec) which is in 

 continuous connection with the ectoderm of the stolon. The inner 

 layer, the entoderm (en), encloses the primary enteric cavity of the 

 bud, which, in Clavelina and the Distomidae, originated as a diverti- 

 culum of the epicardial sac (entodermal stolonic septum, " cloison ").* 

 The connection between the entoderm-vesicle of the bud and the 

 epicardial sac is retained in the social Ascidians (Chivelitw and Pero- 

 phora) for a very long time, often throughout life. According to 

 van Beneden and Julin, the stalk-like portion which connects the 

 hud with the stolonic septum represents the rudiment of the epicardial 

 sac and of the pericardial vesicle of the budding individual. The 

 primary body-cavity extends between the ectoderm and the entoderm 

 of the bud : into it mesoderm-elements soon immigrate, and these are 

 the first rudiment of the mesoderm of the bud. In many cases 

 (especially in the buds of the Distomidae and the Botryllidae) the 



* [In Perophora, according to Rittbb (No. XXVIII.) and Lefevre (No. 

 XXIII.), the developing blastozooid (bud) is connected with the stolonic 

 septum, not by its branchial sac but by the left peribronchial sac. Hitter 

 expresses some doubt concerning the origin of the pericardium ; he thinks 

 that it arises from the inner vesicle, but even if it is produced by the aggrega- 

 tion of mesenchyme-cells, as Lefevre states, it is skill probable that its 

 ultimate source is the entoderm, since the mesenchyme-cells are probably 

 produced from that layer. — Ed.] 



