ANATOMY AND DEVELOPMENT. 20I 



We thus have two solid longitiuHnal mesodermic bands 

 inserted between the ectoderm and endoderm. Anteriorly 

 the mesodermic bands consist of several layers of cells super- 

 imposed one above the other {Y'\\^. 98), but farther back 

 they consist of only one layer of cells. Both portions of 

 the mesoderm — namely, the anterior two- or three-layered 

 and the posterior one-layered portions — arise in continuity 

 with one another, but they have different fates, the former 

 eventually breaking up into loose cells which float about 

 in the body-cavity and constitute the so-called mcsciichyinc, 

 the latter, on the other hand, becoming converted into the 

 musculature of the tail ; whence the former is spoken of 

 as 'Cno. gastral ■i.vi.A the latter as the caudal mesoderm. 



Out (growth of Tail. 



In Amphioxus, at the stage corresponding to that of 

 which we have been speaking — namely, when the embryo 

 has an oval or sub-ellii^tical shape — it bursts through the 

 vitelline membrane insitle which it has already been rotat- 

 ing for some time by means of the cilia of the ectoderm, 

 and escapes into the open sea. This is not the case, 

 however, with the Ascidian embryo. The latter is never 

 ciliated externally, and it remains enclosed within the fol- 

 licular membrane throughout the whole of the embryonic 

 period of development. 



After the stage in question, the growth in the length of 

 the embryo is accompanied by a ventral cicrvature, owing 

 to the confined space in which it is contained. Moreover, 

 the increase in length is not due to a simple elongation of 

 the entire body of the embryo, as is the case with Amphi- 



priinary nicstHlcrni cells ajipcar to ^\^c rise almost exclusively to the cattdaL 

 ntfsodt-rm^ while the gastrnl mesoderm appears to l)e added in front by prolifera- 

 tion from the primitive endoderm as tlescribeti above. 



