A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 315 



tail. It is invested by the test which contains the, as yet 

 wholly unchanged, green structures, and is separated from 

 the vitellary membrane by a space filled with fluid. The 

 substance of the body and of the tail consists of cells ; at least 

 the latter are clearly distinguishable on the surface of these 

 parts. The cells are polygonal, contain granules, and in addi- 

 tion a central nucleus ; the axis of the tail is composed of larger 

 rectangular cells, simply disposed in series, one behind the 

 other, and also provided with a central nucleus, thereby attain- 

 ing a transversely striated or articulated appearance *. 



There are various views as to the mode in which the tail is 

 formed. According to Milne-Edwards the peripheral portion of 

 the embryo, as it were in one piece, becomes constricted off 

 from the body. Kolliker follows this opinion when he states 

 that the tail grows out, not in the manner of a process, but, as 

 a determinate portion of the blastema, becomes separate all at 

 once in its whole length from the body. According to Van 

 Beneden, on the other hand, the tail buds forth as a short 

 process, which elongates by degrees. I can but confirm this 

 last opinion. In fact, according to my observations, the tail 

 appears in the younger embryos, very short and thick in rela- 

 tion to the body, although it is already slightly curved. After- 

 wards it becomes continually longer and more slender, embracing 

 the body in a continually larger arc, until at last it has grown up 

 to such an extent that its terminal portion enfolds the anterior 

 part of the body of the embryo. 



A short time before the completion of the development of the 

 larva, the tail undergoes remarkable changes. The axis becomes 

 hollowed out, its whole cellular substance disappearing into a 

 canal. This excavating process, which is accompanied by a 

 contemporaneous liquefaction of the cell-contents, appears 

 always to proceed from the two walls which are in contact with 

 one another, of every pair of cells, and indeed in many places at 

 the same time, extending further and further, until at last, by 

 the flowing into one another of the separate cavities, the canal 

 in question is formed through the whole length of the axis. In 



* This composition of the axis of the tail of large rectangular cells was first 

 demonstrated by Kblliker in the embryos of Amour oiiclum Nordmanni and 

 Aplidium (1. c. p. 221, fig. 43). 



