ASCIDIACEA FORMATION OP 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 .Il'lix incline bo the first assumption, and Seeliger (No.' 50) 

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

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

 region of the body. 



Fie. 165. — A later stage in the development of Distaplia magnilarva (after Davidoff). 



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



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

 clivme-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 hei'e and in Amphioxus 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 



