200 



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. 100. — Embryo of c/af<f/i«<2/?«- agrees with that of the em- 



soana seen from above, to show the re- 

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

 BENEDEN and juLiN.) A^xivt^ ixom the primitive 



np. Neuropore. en, Endoderm. ent.c. ^ 



Enteric cavity. m.t. Medullary tube, endodcrm, but differs in bc- 

 mei. Mesodermic band. ck. Notochord. . 



ec. Ectoderm. mg solid and unscgmentcd.* 



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

 the Ascidians see \'0N Davidoff's Untersuchungen zur Entuiicklungsgeschichte 

 der DistapHa magnilarva, etc, //. Allgenieine Entw. der Keitiiblatter. jMitth. 

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



As shown by van Beneden and Julin in Clavelina, 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 papulosa 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 plakii/a-stage). 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 



