A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 327 



in the centre of which a solid concretion, in the form of a nucleus, 

 may be remarked. In the larger vesicles the white chalky 

 nucleus appears to be composed of two other rounded segments 

 in close apposition, in the smaller it is perfectly spherical*. 

 According to my observations the vesicles are all completely 

 unconnected with one another; sometimes I thought I could 

 make out a finely reticulated network upon the wall of some of 

 them, without being able to come to any clear conclusion upon 

 the matter. It is the more difficult to decide upon the purpose 

 of the whole structure, as its mode of development, as we shall 

 see immediately, does not throw the desired light upon it. One 

 is tempted to consider it as a purifying organ, a kidney, which 

 would agree very well with the deposits in the vesicles f. In 

 that case, however, we ought to be able to discover excretory 

 ducts, of which I could find not the least trace. 



The first indication of this organ appears when the residue of 

 the tail of the larva, reduced to a very few lobes, is rapidly dis- 

 appearing. Close to these lobes, we see first a small, round, 

 transparent vesicle, already precisely similar in structure to those 

 of the adult organ. The round chalk-white nucleus is already 

 visible in its centre. The vesicle now grows more and more, 

 and at last, when the residue of the tail disappears, it takes its 

 place close to the oesophagus on the left side. Subsequently a 

 second vesicle is formed close to the first, after this a third, and 

 so more and more, until the whole posterior space of the body 

 between the oesophagus, the stomach, and the intestine, is filled 

 up by a mass of such isolated vesicles, in every stage of develop- 

 ment. Afterwards vesicles are seen arising singly and remote 

 from the great mass near certain parts of the alimentary canal. 

 In a few vesicles the nucleus appears double, in others it is pro- 

 vided with the previously mentioned mass of laminated crystals. 

 Whether the vesicles arise at the same time with the nuclei, as 

 we have here described it, or whether the nuclei, as would seem 



* In Phallusia monachus the round nuclei exhibit a close concentric stria- 

 tion, which seems to indicate a deposition in layers. In many vesicles a mass 

 of laminated or acicular crystals lies upon the nucleus ; sometimes one vesicle 

 or another contains, instead of the nucleus, a great prismatic crystal with 

 pyramidally pointed ends. 



t Delle Chiaje {Animali invertehrati d. Sicilia citeriore, t. iii.), to whom these 

 concretions were not unknown, appears to hint at this interpretation. 



