328 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



to be theoretically more probable, are deposited from the fluid 

 of the vesicles at a subsequent period, must be left undecided. 

 I have but rarely seen the nucleus wanting at the first appear- 

 ance of the vesicles. The organ could be traced no further in 

 its development, which is, however, clear enough from what has 

 just been stated. 



Finally, it only remains that I should describe an organ as yet 

 unknown, which is so hidden, as only to be partially visible in 

 the full-grown animal. It consists of a system of fine canals, 

 which are distributed over the whole intestine, commencing in 

 caecal extremities, which are partly cylindrical, partly, and for 

 the most part, enlarged or clavate, and often locally sacculated ; 

 these enter into manifold anastomoses with one another, and 

 thus enclose the wall of the intestine with a close network ; at 

 last they unite into twigs and branches. The canals are found, 

 most abundantly, within the strong projection which runs along 

 the inner surface of the intestine from the stomach almost to the 

 anus, and which is besides filled with the vesicles of the organ 

 which has been already described. The ramification of the 

 branches and twigs, which frequently, during their course, per- 

 fonn arched curvatures somewhat in the manner of the vasa 

 vorticosa of the eye, and in some places dilate into ampullae, is 

 dichotomous ; the contents of the caeca and canals are pellucid. 

 So much for the relations of this organ in the adult animal ; 

 development affords us the following further information. 



Very early, even before the appearance of the first gill-clefts 

 and of the heart, there appears at the beginning of the intestine 

 immediately behind the stomach, a cylindrical process, alto- 

 gether homogeneous, and with a somewhat enlarged, clavate, free 

 extremity, which passes transversely, on the left side, to the vici- 

 nity of the terminal portion of the intestine. Without percep- 

 tible alteration, this process continues slowly to grow, until at 

 last we see it divide at its extremity into many branches, which 

 pass to and embrace the intestine. 



Very soon these branches also divide, dichotomously, into 

 twigs, which by their frequent anastomosis form a network enclo- 

 sing the intestine. In this manner the ramification proceeds, the 

 network upon the intestine becomes closer, and at last the caeca 

 above referred to make their appearance upon it. The branches 



