STUDIES ON THE PROTOCHORDATA. 329 



cavity in the prseoral lobe would, however, be better described 

 as the anterior body-cavity. 



In a young larva the endoderm is in contact anteriorly with 

 the ectoderm ; but in a rather older larva the anterior ecto- 

 derm bearing the adhering papillae springs away from the en- 

 doderm, leaving a space which becomes filled with a compact 

 mass of mesoderm-cells (PL XXX, fig. 1). 



The mesoderm which comes to lie in this anterior body- 

 cavity has a bilateral origin, in that it proceeds from the two 

 lateral mesodermic plates. Fig. 15 shows a transverse section 

 through the prseoral region of a larva of the stage of fig. 1, 

 cutting the endostyle vertically; and on either side of the 

 latter is seen a group of mesoderm-cells, which form part of 

 the lateral mesodermic bands. The latter pass directly into 

 the mesoderm of the anterior body-cavity. 



In Distaplia magnilarva, Davidoff (8) has found that a 

 portion of this anterior mesoderm is derived directly from the 

 anterior extremity of the endoderm, from which cells are given 

 off into the body-cavity. 



The mesoderm so produced is called by Davidoff the prse- 

 gastral mesoderm. The occurrence of this prsegastral 

 mesoderm in Distaplia is in itself, apart from other considera- 

 tions, a serious argument against van Beneden's and Julin's 

 theory of the prechordal vesicle of the Ascidian embryo and 

 its homology with that portion of the enteric cavity of Am- 

 phioxus from which the anterior intestinal diverticula arise, as 

 will appear from the following definition which they give of 

 their prechordal vesicle. They say (7, p. 385), ^^Le meso- 

 blaste et la notochorde se forment aux depens de Tendoderme 

 primitif de la Gastrula, dans toute la longueur de la larve, h, 

 I'exception de son extremite anterieure. L'on pent done dis- 

 tingueur dans la longueur du corps de la larve une partie an- 

 terieure prechordale dans laquelle lachordeet le mesoblaste 

 font defaut et une portion chordale, dans les limites de 

 laquelle s'etendent la notochorde et les bandes mesoblastiques. 

 II en resulte aussi que, chez I'Amphioxus comme chez la 

 Claveline, le tube digestif des jeunes larves se constitue de 



