356 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



the dorsal cells of that portion of the embryonic nervous system which 

 has a thickened ventral wall. The remainder is formed from this thick- 

 ened portion of the ventral wall of the neural canal. The dorsal part 

 of the ganglion would seem, then, to be certainly homologous with the 

 ganglion of ascidians and pyrosoma ; it being a structure of secondary 

 formation in each and having similar origin. In doliolum, if we had 

 full data for judging, it is probable a similar origin would be found for 

 the dorsal part of the adult ganglion. The almost complete resemblance 

 between the immature salpa's nervous system and that of the nearly 

 mature doliolum argues strongly in favor of this. 



3. The relation of the dorsal and ventral portions of the immature 

 salpa's nervous system to the duct that opens into the funnel is much 

 the same as the relation of the ganglion and subneural gland to the duct 

 of that gland in the older Iarva3 of many ascidians, e. g., amaroecium. 

 In this connection the lacunae in the immature salpa ganglion, opening 

 into the central canal of the nervous system and, on the other hand, the 

 connection between the larval ascidian ganglion and the duct are sug- 

 gestive. 



4. The caudal portion of the larval ascidian nervous system degen- 

 erates. In salpa there is an absence of any caudal portion of the nervous 

 system, correlated with the absence of any normally developed tail. 



5. The sense vesicle of the larval ascidian is not present in any 

 recognizable form in either salpa, doliolum, or pyrosoma at any known 

 stage of their development. It seems to be a special larval character 

 secondarily acquired by the ascidians. (See also the last paragraph of 

 Appendix I.) 



6. The ventral wall of the visceral portion of the ascidian tadpole's 

 nervous system proliferates cells to form the body of the subneural 

 gland ; the whole ventral wall being apparently thus used up. In salpa 

 most of the thick ventral wall persists to form the ventral third of the 

 ganglion, but certain cells push out ventrally toward the hollow disks of 

 the subneural gland, recalling the similar, though more extensive, migra- 

 tion of "nerve-gland" cells in the ascidian larva. 



If the ganglia of salpa and the ascidians be homologous, as seems so 

 evident, the three so-called vesicles described by Salensky in the imma- 

 ture ganglion of Salpa scutigera-conf ederata lose the importance assigned 

 them by Salensky. They are then to be regarded as secondary, and 

 having no morphological connection with the three primary vesicles of 

 the vertebrate brain, or with the three regions of the neural canal in the 

 ascidian tadpole. 



