ASCIDIACEA FORMATION OF THE GERM-LAYERS. 



351 



here pressed inward by the developing medullary tube, so that it 

 then fills the whole lumen of the intestine. The lateral cells of the 

 wall of the eiiteron, three of which are usually seen on either side 



FIG. 163. Later stage of development of Clavdina Rissoana (after VAN BBNBDEN and 

 JULIN). A, median sagittal section ; ft, lateral aspect ch, chorda ; ec, ectoderm ; 

 en, entoderm ; en', subchordal entoderm-strand in the caudal region ; ms, anterior 

 small-celled portion of the mesoderm-bands ; ms', posterior large-celled caudal 

 section of the same ; n,)>, neuropore ; nr, medullary tube. 



in cross-section, pass directly into the large mesoderm-elements 

 which cover the chorda laterally and subsequently yield the caudal 

 musculature. There then still remain the entoderm-cells that lie 



