STUDIES ON THE PROTO CHORD ATA. 343 



not regard the fact that the atrial cavities fuse together 

 dorsally in the Ascidians, and ventrally in Amphioxus, as a 

 valid objection to their complete homology in the two groups. 

 Van Beneden and Julin (6) lay great stress on the cloaca of 

 the Ascidians being a special formation, produced not only by 

 the concrescence of the two primary atrial apertures, but also 

 by a new depression in the dorsal wall of the body, and, in 

 contrast to the atrial chambers, being delimited on all sides 

 by ectoderm. They say that before their fusion the two atrial 

 apertures do not project from the body of the Ascidian in the 

 form of siphons. I must, however, confirm Krohn (23) in 

 saying that they do. The special dorsal depression, however, 

 might very well occur, although I have not been able to see it ; 

 but the contrast to the atrial chambers in regard to the deri- 

 vation of the epithelial lining of its walls does not, as we have 

 seen above, exist. 



6. Intestine and Caecum of Ascidians and 

 Amphioxus. 

 Since, according to van Beneden and Julin the prechordal 

 vesicle, including the first trunk somite of the Ascidians, had 

 enlarged itself at the expense of the trunk generally, so as to 

 contain in itself all the organs which usually belong to the 

 trunk proper, it followed among other things that the intes- 

 tine of Ascidians was a new formation, a special outgrowth 

 from the posterior end of the prechordal portion of the enteric 

 cavity, and was not homologous with the intestine of Amphi- 

 oxus. The tail of the Ascidian tadpole represented for them 

 the trunk of Amphioxus, and the solid cord of endoderm-cells 

 arranged in a double series, each series being composed of 

 single cells placed end to end and lying immediately below 

 the notochord, which is a constant feature of the tail of the 

 Ascidian tadpole, was for them the rudiment of a primitive 

 intestine which formerly extended to the end of the tail, and 

 there discharged by the anus. It might, at first sight, appear 

 that this view of a rudimentary intestine in the tail of the 

 Ascidian tadpole had certainly much to be said for it on mor- 



