468 



TUNICATA. 



The rudiment of the genital organs appears dorsally to the intes- 

 tinal loop immediately below the nerve-strand (Figs. 238 E, </, and 

 237, g). The strictly median position of this rudiment is specially 

 distinct in the buds of Glavelina, where it appears only in later 

 stages. It apparently takes its rise as an agglomeration of closely 

 crowded mesenchyme-cells, in which a cavity nevertheless soon 

 appears ; this is the primary genital cavity round which the cells 

 become grouped like an epithelium. For the development of the 

 male and female genital organs from this uniform rudiment we must 

 refer the reader to p. 379. 



/ 



M 



C 



Fig. 239. — Three consecutive ontogenetic stages in the so-called bud of Ama/rcmcium 

 (after Kowalevsky). A, dorsal view of the anterior part of the body ; B, side view 

 of an older stage ; C, dorsal view of a still more advanced stage, d, enteric rudi- 

 ment ; ec, ectoderm; en, entoderm; ej>, epicardial process; », rudiment of the 

 nervous system ; p, peribranchial sac. 



Important observations on the condition of the pericardial vesicle 

 and the epicardial tube have been made by van Beneden and Julin. 

 Following these authors, we shall first describe the condition of a 

 more fully developed bud traced in a series of cross-sections. The 

 last section of this series (Fig. 238 F) passes through the base of the 

 U-shaped intestinal loop (i) and, ventral to this, the stolonic septum 

 (st) is seen. In anterior sections, we rind that the latter is in direct 

 connection with the pericardial vesicle (Fig. 238 D, pc). The heart 

 (h) here also has arisen through the invagination of the wall of the 

 pericardial sac (pc). A section cut further forward (Fig. 238 D) 

 shows the double or forked end of the epicardial sac (ep) of the bud, 

 in close contact with the pericardial vesicle. Further forward, the 

 pericardial vesicle decreases in size (Fig. 238 C, pc) and finally 



