2OO 



THE ASCIDIANS. 



ference between the Ascidian and Amphioxus. The 

 mesoderm in the Ascidian embryo does not arise as a 

 series of archenteric pouches, but is produced on each side 

 by a solid proliferation of cells from the primitive endoderm 

 which lines the archenteric cavity. This solid proliferation 



begins in the middle region 

 of the embryo near the an- 

 terior limit of the notochord, 

 and extends backwards (Figs. 

 98 and 100). It takes place 

 from the dorso-lateral cells 

 of the endoderm, in a posi- 

 tion corresponding to that 

 at which the mesoblastic 

 pouches of Amphioxus grow 

 out from the archenteron. 

 The mesoderm of the As- 

 cidian embryo therefore 

 Fig. TOO. Embryo of ciaveiina Ris- agrees with that of the em- 



soana seen from above, to show the re- 

 lation of parts. (Simplified after VAN bryo of Amphioxus in being 



BENEDEN and JULIN.) derived f h primit i ve 



np. Neuropore. en. Endoderm. ent.c. 



Enteric cavity. m.t. Medullary tube, endoderm, but differs in be- 



mes. Mesodermic band. ch. Notochord. . 



ec. Ectoderm. ing solid and unsegmented.* 



* For a recent and elaborate discussion of the origin of the mesoderm in 

 the Ascidians see VON DAVIDOFF'S Untersuchungen zur Entwicklungsgeschichte 

 der Distaplia magnilarva, etc., II. Allgemeine Entw. der Keimblatter. Mitth. 

 Zool. Stat. Neapel, IX. 1891. pp. 533-651. 



As shown by van Beneden and Julin in Ciaveiina, the primary mesoderm 

 of the Ascidian embryo can be detected at a much earlier stage of development 

 than in Amphioxus. 



I have studied the origin of the mesoderm in Cynthia papillosa and found 

 that the primary mesoderm cells are to be distinguished, by their poverty in 

 food-yolk, from the remaining endoderm, at the commencement of gastrula- 

 tion (at the so-called plakula-stagi). They occur in the form of a crescent 

 round the posterior margin of the blastopore, and are carried in by the invagi- 

 nation, and then increase in number by mitotic division. In Cynthia, these 



