518 



Mr. A. Willey. On the Post- Embryonic [May 19, 



The walls of the atrial chamber are apparently derived essentially 

 from the ectoderm. 



The first pair of gill-slits which becomes sub-divided in the re- 

 markable way above described (totally different from the transverse 

 snb-division which they subsequently undergo), is present in a simple 

 undivided form in Appendicularia, and I consider it, both from its 

 position and from its specialised character, to be undoubtedly homo- 

 logous with the first pair of gill-slits in Amphioxus, which disap- 

 pear at the close of the larval period. Both in Ciona and in 

 Amphioxus this first pair of slits alone serves for the respiration of 

 the larva (or, in the case of Ciona, young individual) for several 

 weeks, during which the size of the animal is increased, but no new 

 organs are added. In other words, there is a resting stage in the 

 development of Amphioxus and Ciona, which is characterised in both 

 cases by the presence of a single pair of gill-slits, namely, the first 

 pair. 



By the first pair of gill-slits of Amphioxus, I refer to the first gill- 

 slit proper, and to the club-shaped gland. I have on a former occa- 

 sion given cogent reasons for regarding these two structures as a pair 

 of gill-slits. 



In Ciona, at a later stage, the primary stigmata, whose origin has 

 just been described, become divided in the usual way and give rise to 

 the transverse rows of stigmata. In Clavelina these transverse rows 

 of stigmata form in the first instance, each aperture being an independent 

 perforation. Here, then, we have unequivocal evidence of consider- 

 able modification in the direction of a shortening of the develop- 

 ment in the case of Clavelina ; and we meet with similar evidence at 

 every turn. 



Origin of Pericardium and Heart in Ciona and Clavelina. 



With regard to the origin of the pericardium, my observations 

 appear to confirm, in the main, the account given by van Beneden 

 and Julin as to its being endodermic ; but, as I have never succeeded 

 in finding karyokinetic figures in connexion with its development, a 

 precise statement as to its mode of appearance is not at present 

 possible. 



In Clavelina, where it is comparatively easy to be persuaded of its 

 endodermic origin, it arises at a much earlier stage in the development 

 of the larva than it does in Ciona. In the former it arises before the 

 formation of the body-cavity, while in the latter there is a wide body- 

 cavity present at the time of its first appearance, containing loose 

 mesoderm cells ; and the failure to find nuclear spindles, combined 

 with the extraordinarily small size of the object in transverse section, 

 renders it extremely difficult to assign its origin to the endoderm 



