ANATOMY AND DEVELOPMENT. 2OI 



We thus have two solid longitudinal mesodermic bands 

 inserted between the ectoderm and endoderm. Anteriorly 

 the mesodermic bands consist of several layers of cells super- 

 imposed one above the other (Fig. 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 mesenchymc, 

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

 musculature of the tail ; whence the former is spoken of 

 as the gastral and the latter as the caudal mesoderm. 



Outgrowth 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-elliptical shape it bursts through the 

 vitelline membrane inside 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 curvature, 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- 



primary mesoderm cells appear to give rise almost exclusively to the caudal 

 mesoderm, while the gastral mesoderm appears to be added in front by prolifera- 

 tion from the primitive endoderm as described above. 



