Budding in Perophora. 143 



themselves into an epithelium, and the structure has now the 

 form of a long tube closed at both ends. 



By this time tlie tube has been carried up to the dorsal 

 mid-line by the shifting of the inner vesicle, and later its 

 anterior extremity fuses with the dorsal wall of the branchial 

 sac, whereupon an opening breaks through to put the lumen 

 of the tube in communication with the branchial cavity. 



In the preliminary note, referred to at the beginning of this 

 article, the statement was made that " the ganglion is formed 

 by a thickening of the dorsal wall of the tube, which even- 

 tually becomes constricted off." More careful study of very 

 young stages has shown that this is not a correct description 

 of the formation of the ganglion. 



About the time that the dorsal tube acquires' its opening 

 into the pharynx a few cells, identical in appearance with the 

 mesodermal cells, are found adhering to the dorsal surface of 

 the tube along nearly the whole length of the latter. It is a 

 difficult question to decide whether these cells are meso- 

 dermal, as their appearance indicates, or whether they are 

 wandering cells from the wall of the tube, for in many places 

 the border of the latter is broken, and there is a decided indi- 

 cation of migration of cells into the rudiment. On the other 

 hand, it is perfectly evident, especially in later stages, that 

 free amoeboid cells are added to the mass from without. The 

 conclusion has therefore been drawn that the ganglion has a 

 double origin and that both the dorsal wall of the tube and 

 free mesodermal cells co-operate in forming it. 



The rudiment increases enormously in size, the cell- 

 boundaries are completely lost, and the nuclei arrange them- 

 selves peripherally into two or three layers around a central 

 core in which fine fibrils are afterwards laid down. The 

 definitive structure of the ganglion is then attained. 



The Sexual Organs. 



Our observations on the development of these organs closely 

 agree with the description given by Van Beneden and Julin * 

 for the buds of Perophora Listeri, 



Shortly after the peribranchial cavity has been completely 

 separated from the brancliial sac a small collection of meso- 

 dermal cells is found lying in front of the wall of the stomach 

 almost at the point wliere the latter is connected with the 

 duct of the " organe reiringent." A cavity soon appears in 



* Arch, de Biolo'ne, t. vi., 1837. 



