WIDIACEA FORMATION OF THE (iKKM- LAYERS. 



349 



origin from folds and is stated by Seeligeb (No. 50) as occurring in ClaveUna 

 also, but his observations on this subject were not confirmed by van Benedkn 

 and Julin (No. 10). These latter authors also do not agree in Seelig i ib - 

 view thai the medullary groove, at the time when it appears, lengthens 

 posteriorly beyond the blastopore. 



At the time when the medullary tube develops, the blastopore has not com- 

 pletely closed. The remains of it, which originally lie in the floor of the de- 

 veloping medullary groove, are retained for some time longer as the neurenteric 

 ritual and form a communication betweeen the lumen of the intestine and the 

 central canal of the medullary tube (Fig. 158 B). 



— Tip 



■nr 



ms 



FlG. 162. — Stage at which, in ClaveUna Rissoana, the trunk-region begins to separate 

 from the caudal region (after van Beneden and Juun). .1. median sagittal section ; 

 /i, lateral aspect, ch, chorda; d, archenteric cavity; ec, ectoderm ; en, entoderm ; 

 en', sub-chordal entoderm-strand ; ms, anterior portion of the mesoderm-haii'ls 

 composed of small cells ; ,,/s', posterior portion of the same composed of large cells ; 

 rip, neuropore; ///-, medullary tube. 



AlS the medullary tube develops from behind Forward, the aperture 

 at its anterior cud, known as the neuropore (Fig. 162, np), is retained 

 for a long time. The separation of the mesoderm from the chorda 

 dorsalis takes phice simultaneously with the development of the 

 medullary tube. These two rudiments arise, as may be ascertained 

 from the detailed accounts of van Beneden and Julin, essentially 

 through the same processes of development as in Amphioxus, although 

 the conditions are in this case not so evident, and seem specially 

 modified in the posterior region of the body. The embryo soon 

 assumes a long, pear-shaped form (Fig. 162), the posterior, narrowed 



