ASCIDIACEA FORMATION OF THE GERM-LAYERS. 



353 



shall have here to assume (with VAN BENEDEN and JULIN) in these regions, 

 the presence of a lumen of the archenteron compressed through the growth of 

 the chorda-rudiment to the shape of a crescent. The question here arises 

 whether the mesoderm-layer of the caudal section is to be referred to the 

 splanchnic or the somatic layer of the former mesoderm-rudiment. VAN 

 BENEDEN and JULIN incline to the first assumption, and SEELIGEB (No. 50) 

 has also pointed out the resemblance of the mesoderm-cells of the caudal 

 section to to the cells of the inner layer of the mesoderm in the anterior 

 region of the body. 



n>r 



\ 



c, caudal prolongation of the alimentary canal ; ch, rudiment of the chorda ; 

 d, enteric cavity ; ec, ectoderm ; en, entoderm ; h, adhesive papillae ; ms, mesen- 

 chyme-cells ; nr, medullary tube. 



The mesoderm and the chorda are therefore derivatives of the 

 primary entoderm.* Their origin can be traced back to the form of 

 folding which is also found in Amphioxus. The principal distinction 

 between the process here and in Amphioxu* seems to be that, in the 

 mesoderm-rudiment of the Ascidians, no trace is to be found of the 

 segmentation which appears so early in Amphioxus. In the Ascidians 

 the mesoderm-bands appear composed of two different parts (Figs. 

 162 B, 163 B) ; an anterior part (ms), consisting of several layers of 

 small cells, which has arisen through folding in the anterior part of 

 the archenteron, and a posterior part composed of a single layer of 

 large cells (ms') belonging to the caudal region. The anterior part of 

 the mesoderm, at a later stage, forms a mesenchyme filling the 



* [See footnote, p. 340. ED.] 

 AA 



