ASCIDIACEA FORMATION OF THE GERM-LAYERS. 



349 



origin from folds and is stated by SEELIGEB (No. 50) as occurring in Clavelina 

 also, but his observations on this subject were not confirmed by VAN BENEDEN 

 and JULIN (No. 10). These latter authors also do not agree in SEELIGER'S 

 view that 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 

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

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



..-Up 



B 



ch 



Fi<;. 162. Stage at which, in Clavelina Rissoana, the trunk-region begins to separate 

 from the caudal region (after VAN BENEDEN and JULIN). A, median sagittal section ; 

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

 <//', sub-chordal eutoderni-straiul ; ms, anterior portion of the mesoderm-bands 

 ' -(imposed of small cells ; ins', posterior portion of the same composed of large cells ; 

 n/>, neuropore ; nr, medullary tube. 



As the medullary tube develops from behind forward, the aperture 

 at its anterior end, known as the neuropore (Fig. 162, np\ is retained 

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

 '/vrsdlis takes place 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 



\ms 



