348 



TUNICATA. 



posteriorly in the form of a semicircle (Figs. 155 and 158). It then appears, 

 especially in longitudinal sections (Fig. 158 B), as if the medullary tube was 

 formed solely by the posterior lip of the blastopore growing over the anterior 

 lip. We must, however, bear in mind that, as this posterior medullary fold 



FIG. 160. Transverse sections through an embryo of Clavelina Rissuana, at the same 

 stage as in Fig. 158 (adapted from VAN BENEDEN and JULIN). A, through the. 

 anterior, B, through the middle, and G, through the posterior part of the body. 

 ch, chorda ; d, lumen of intestine ; en, entoderm ; n, medullary plate ; ///, medullary 

 tube. 



grows forwards, the lateral medullary swellings are drawn into it, so that the 

 posterior fold does not actually represent the lip of the blastopore, but a purely 

 ectodermal fold lying at this point ; in this way, after the medullary tube has 

 developed, its roof (ri) which is derived from the inner layer of the folds is 

 also to be regarded as ectodermal. This is a point which deserves to be 

 emphasised in opposition to the view of METSCHNIKOFF (No. 42). [Cf. foot- 

 note, p. 345.] 



FIG. 161. Transverse sections through an embryo of Clavelina Rissoana, at the same 

 stage as in Fig. 162 (adapted from VAN BENEDEN and JULIN). A, through the 

 anterior, B, through the middle, and C, through the posterior section of the body. 

 ch, chorda; en, entoderm; ms, mesoderm ; nr, medullary "tube; x, cells which com- 

 plete the dorsal wall of the intestinal canal. 



We shall see (Chap. XXXVI.) that, in Amphioxus, the medullary tube forms 

 . by the sinking in of the somewhat depressed medullary plate and its separa- 

 tion as such from the ectoderm ; hence it is only covered by a single layer 

 of cells. Only at a later stage does the plate curl round under the ectoderm 

 to form a tube. This manner of formation is probably a modification of the 



