350 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



between the two was secondarily established. If the former hypothesis 

 be the true one, pyrosoma, doliolum and salpa retain or have reverted 

 to the primitive condition which the ascidian larva, according to Yan 

 Beneden and Julin, has lost. If the latter hypothesis be correct, the 

 early appearance of the communication between the cavity of the 

 nervous system and the pharynx in these three genera is an instance 

 of accelerated development. (See Appendix I.) 



Let us now turn to the chain form of Cyclosalpa pinnata and observe 

 the development of the subneural gland and the organs connected with 

 it. In an early stage of the development of the nervous system, long 

 before any trace of the eye appears, the cavity of the brain and the 

 lumen of the funnel open freely into each other by a wide duct, so short 

 and wide as to hardly deserve the name duct. There is no distinction in 

 histological character between the cells of the funnel, duct and brain ; nor 

 is there any indication either in the thickness of the walls or in any 

 other feature of the boundaries between funnel, duct and brain. The 

 ventral wall of the posterior part of the neural canal becomes thickened. 

 This corresponds, as will be shown later, to the thick ventral wall of the 

 " visceral portion " of the larval ascidian nervous system, i. e., that por- 

 tion which gives rise to the subneural gland. Later, the cells of the 

 dorsal wall of the posterior part of the neural canal begin to multiply 

 rapidly ; some of them keeping their original arrangement as an epithe- 

 lium bounding the canal ; others, much more numerous, push up toward 

 the ectoderm to form the dorsal portion of the adult ganglion. This 

 proliferation of cells is greatest, for a time, at the most posterior part of 

 the dorsal wall of the neural canal. The brain cavity has now a thick 

 ventral wall, and a dorsal wall about twice as thick as the ventral. Fig. 

 4, Plate LI, shows one stage in the development of the ganglion of the 

 solitary salpa. It will answer equally well for the chain individual. 



At the time of the first appearance of the rudiment of the dorsal eye 

 the three regions, funnel, duct and brain, are clearly distinguished, the 

 duct being a small round tube connecting the lumen of the funnel with 

 the cavity of the brain (Plate XLVII, Fig. 5). The description so far 

 would answer almost equally well for either salpa, doliolum or pyrosoma, 

 except that the latter two do not have the ventral wall of the neural 

 canal thickened. Fig. 4, Plate XLVII, shows an oblique section through 

 the duct at this stage. The lumen of the duct is cut in two places, at / 

 near the brain where it is larger, at / x nearer the funnel where it is 

 smaller and thinner-walled. The brain cavity at this time is much 



