454 



On the Origin of the Ascidian Mouth. 



By A. G. Huntsman, B.A., M.B., Biological Department, University of 



Toronto. 



(Communicated by Prof. A. B. Macallum, F.B.S. Received March 3, — Bead 



April 24, 1913.) 



A year ago I undertook to determine the origin of the test-bearing and 

 testless parts of the epithelial lining of the siphons of Ascidians. My 

 material consisted of adult Dendrodoa (Styelopsis), Cccsira [Ilolgula], and 

 Clavclina, each containing embryos of all stages. These were studied in 

 serial sections. 



In the case of the oral siphon, I reached the unexpected result that a large 

 part of the epithelium of the wall of the oral cavity is derived from the 

 primitive neural tube of the embryo. It can readily be understood that the 

 connecting of definite parts of the epithelium of the adult with definite parts 

 of the germ layers of the embryo cannot be dene with any great certainty. 

 For several reasons I conclude that the outer limit of the " neural " epithelium 

 of the oral siphon is the margin of the test-bearing epithelium (indicated 

 usually by a distinct ridge or velum), and that the inner limit is the 

 peripharyngeal band or ridge. 



The mouth of the Ascidian corresponds strictly with the neuropore of the 

 embryo. The neuropore closes rather early, but re-opens during or after 

 metamorphosis to form the connection between the test-bearing and testless 

 parts of the siphon. 



In the three genera investigated I have been unable to find any stage in 

 which the neural tube is separated from the ectoderm at its anterior end, as 

 maintained by Kupffer, Kowalevsky, and Seeliger. I have examined 

 numerous stages (both a year ago and recently), from that of the open 

 medullary tube to the escaped larva, and in all the neural tube is connected 

 with the ectoderm. 



The accounts that have been given of the manner in which the stomodeeum, 

 the hypophysial canal, and the pharynx become connected are more or less 

 conflicting. 



Kupffer (1870) failed to find a neuropore at any stage. He describes the 

 fusion of a stomodseal invagination, with the pharynx to form the mouth. 



Kowalevsky (1871) describes the early closure of the neuropore, and 

 derives the mouth from an invagination of the ectoderm, which connects 

 almost simultaneously with the sensory vesicle of the neural tube and with 

 the pharynx. 



