346 



TUNICATA. 



side of which, however, as the blastopore narrows, soon extends to 

 form a cell-area in front of it. The whole of this area represents the 



FIG 156 Transverse sections through two gastrula-stages of Distaplia magnilarva 

 (after DAVIDOFF). A, younger stage : B, transverse section through the posterior 

 part of an older stage, b, blastopore ; ec, ectoderm ; en, entoderm ; n, nerve-ring. 



common rudiment of the mesoderm and the chorda, the " chorda- 

 mesenchyme " ring. The middle cells of this area soon become more 



distinctly separated from 

 the rest, and represent the 

 plate-like rudiment of the 

 chorda (Fig. 158, ch), while 

 the cells that lie laterally 

 give rise to the mesoderm- 

 bands (ms).* 



In the eggs of the composite 

 Ascidians, which are rich in 

 yolk, the formation of the 

 germ-layers is to a certain ex- 

 tent modified. The cleavage- 

 cavity which, in Phallusia, 

 is rather large, is smaller in 

 Clavelina, while in Distaplia 

 (according to DAVIDOFF) it 

 can only be seen in the first 



FIG. 157. Transverse section through an embryo 

 of Distaplia magnilarva after the closing of the 

 blastopore (after DAVIDOFF). ec, ectoderm ; en, 

 entoderm ; ins, mesoderm ; n, neural plate. 



* [In Ciona, according to CASTLE, the mesoderm-rudiment is made up of cells 

 derived from both hemispheres and all four quadrants, a condition very 

 different from that seen in the Mollusca, where the mesoderm typically arises 

 from the left posterior entodermal macromere. ED.] 



