BANCROFT : OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 85 



erally accepted from that time ; but quite recently a vigorous supporter 

 of the older theory has been found in Salensky ('92, '94, pp. 441-449, 

 '95, pp. 618-G21). 



This author, finding that in Salpa and Pyrosoma cells which are de- 

 rivative of the follicle take a large part in the formation of the embryo, 

 turns to the compound ascidians expecting to find there the beginnings 

 of the same process. Accordingly, he makes out that in the compound 

 ascidians the test cells, or kalymmocytes, as he calls them, have a func- 

 tion to perform during the life of the embryo, though this differs in 

 different cases. In representative species of the genera Fragarium, 

 AmaroDcium, and Circinalium he found a placenta attaching the embryo 

 to the wall of the cloaca, and the foetal part of this structure was de- 

 rived from transformed test cells. In Distapli^ magnilarva, Diplosoma, 

 and Didemnum these cells have the function of forming the test of the 

 larva, the whole of it being derived from the test cells in Diplosoma, but 

 only a part of it in the other two genera. 



In Distaplia, which is the only genus in which this process need 

 concern us, Salensky finds that at the time when the test is forming, 

 the test cells flatten out against the ectoderm of the larva so that 

 in places they form continuous layers. These he thinks secrete the 

 cellulose matrix ('92, pp. 113-114) and then metamorphose into the 

 cells lining the lacunar spaces with which the test is honey-combed. 

 The test, he thinks, continues to grow in the same way, having layer 

 after layer of test cells added to its outer surface. Caullery ('94, 

 p. 600), the only other author who has studied this subject since the 

 publication of Salenky's first article ('92), comes to the conclusion that 

 the test cells take no part in the formation of the test, but does not 

 give any evidence. 



In examining Distaplia occidentalis, an appearance was quite fre- 

 quently noted that looked very much as if there were unmodified test 

 cells within the test itself. But when examined more carefully, the 

 substance within which the test cells were contained was found at stages 

 before there was any test being formed, and even before the egg had 

 segmented. It was also found within the cavities of some of the older 

 embryos, and was then concentrated on the same side of all the cavities. 

 If in these cases the substance was situated on the left side of the 

 cavities, then that on the outside of the embryo was always on the 

 left side and vice versa. Thus there was every reason to believe that 

 the substance surrounding the test cells was a coagulum that had 

 been thrown down by the fixing reagents, and had reached its character- 



