548 MAYNARD M. METCALF, 



ganglion in the Salpidae is the homolog of the neural gland of Ascidians. 

 At an early stage in the development of the embryos and buds of 

 Salpa the nervous system is a compact mass of cells enclosing a slit- 

 like cavity which opens in front into the funnel-shaped neuropore, not 

 yet ciliated (Fig. 75). Below the slit-like cavity the cells of the 

 nerve tissue are very numerous and crowded, making a thick solid 

 mass, homologous to the thickened part of the wall of the nerve tube 

 in Ascidians, from which the neural gland develops. Above the slit- 

 like cavity there is a thin layer of compactly arranged cells that are 

 giving off dorsally cells which, as they increase in number, give rise 

 to the brain. This evidently corresponds to the formation of the 

 brain in the Ascidians from the thin part of the wall of the nerve 

 tube, opposite to the gland. In the Ascidian (or Pyrosoma or Boli- 

 olum) the brain is formed from only one wall of the larval nerve tube, 

 the other wall forming gland tissue. In the Salpidae the brain is 

 formed from both walls of the embryonic nerve tube, so that the 

 brain of Salpa is homologous with both the brain and the gland in 

 Ascidians ^). It is remarkable to find at all, as we do in the Tuni- 

 cates, a gland arising by the transformation of nerve cells. It is 

 still more remarkable to find in some species of Tunicates (the Sal- 

 pidae) the homologous nerve cells not giving rise to gland tissue, but 

 remaining as part of the definitive brain. 



The conditions here described and the facts as to the origin of 

 the ganglion cells of the rapheal nerve in Ascidians, in some species 

 from the brain, in others from the neural gland, and in others from 

 both brain and gland, show an intimacy of relation between nerve 

 tissue and glandular tissue, hardly to be paralleled elsewhere in the 

 animal kingdom (cf. page 531). 



Salpa, however, does have a neural gland consisting of two cham- 

 bers ventro-lateral to the brain, each connected by a duct with the 

 pharynx (or cloaca?). These organs arise in the ontogeny from the pha- 

 ryngeal (or cloacal?) wall. Are there any structures in the Ascidians, 

 Pyrosoma or Doliolum, to which these may be related ? In Pyrosoma and 

 in most Ascidians I find nothing. In Doliolum, whose neural gland I have 

 been unable to study, it is possible we have in the ventro-lateral 

 processes from the gland (0 in Fig. 55, Plate 38) something cor- 

 responding to the ventro-lateral chambers of the gland in the Salpidae. 



1) Cf. Metcalf, 18952, p. 355—357. 



