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 



en 



ec 



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- 

 1116861101171116' ' ring. The middle cells of this area soon become more 



n 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 

 Clarclina, 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; ms, mesoderm ; it, 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.] 



