410 Miscellaneous. 



and finally M. Julien, after describing its structure in various As- 

 cidia with great care, has regarded it, iu agreement with M. E, Van 

 Beneden, as representing the hypophysis of the Vertebrata. 



The investigations which I have been making for two winters 

 upon the anatomy and gemmation of Pi/rosoma, some results of 

 which I have already communicated to the Academy, have led me 

 to study the structure and formation of the ganglion and the pit in 

 that Tunicate. It seems to me that the facts that I have been able 

 to ascertain must throw some light on the question. 



The ciliated pit or sac of Pi/rosoma has been very well described 

 by Huxley. According to that author it consists of an elongated 

 canal applied along the median line against the branchial surface of 

 the ganglion, probably terminating ctecally behind, opening in front 

 into the branchial sac by a scarcely dilated orifice, and presenting a 

 small projecting tubercle in its middle region. 



I may add that the walls of the canal are formed by a cubical 

 epithelium destitute of cilia, that a few cilia and two or three flagella 

 occur only quite at the entrance, at the point of union with the bran- 

 chial sac, and that the median tubercle is formed by an aggregation 

 of small rounded cells arranged around a diverticulum of the canal. 



The whole organ evidently represents the duct of the gland of the 

 Ascidia ; the anterior ciliated part corresponds to the vestibule ; and 

 the median tubercle seems to me to represent a rudimentary gland. 



In describing the formation of the vibratile pit in the bud of Py- 

 rosoma, Kowalevsky expresses himself as follows: — " The wall of 

 the branchial sac forms a small depression, which represents the 

 first trace of the vibratile pit ; this pit sinks a little into the gan- 

 glion, which consists of an aggregation of cells. At this period the 

 nervous si/atem has lost its primitive canal-like form, and consists of 

 an elongated aggregation of rounded cells, in the midst of which we 

 no longer perceive more than a feeble indication of the original 

 cavity." This description, as we shall see, is very far from the 

 truth ; for the ganglion, properly so called, does not present a cavity 

 at any moment of its existence, while the primitive neural canal 

 retains its cavity, which is nothing else than that of the ciliated sac. 



The section of the very young bud given by the Russian natura- 

 list is correct ; and I have been able to ascertain that what he calls 

 the first trace [or rudiinent'] of the nervous system, represented at 

 the base of the stolon by a simple train of cells, becomes converted 

 a little later into a canal ; the constriction which separates the 

 future zooids from one another finally converts it into a pyriform 

 vesicle. 



Does this vesicle afterwards become obliterated to form the gan- 

 glion, as supposed by Kowalevsky ? By no means ; it continues to 

 enlaro-e for a long time ; its cavity dilates and its walls thicken 

 gradually. Subsequently, and in buds which are already advanced, 

 its posterior wall thickens still more, and from it separate some 

 round cells which are placed between the vesicle and the ectoderm. 

 The posterior wall then resumes its original thickness, and remains, 



