ASCIDIACEA — FORMATION OF THK GEKM-LAYKKS. 



347 



stages as a narrow slit. The entoderm-cells also, in tltis latter form, do not long 

 retain their unilaminar arrangement, hut become distributed in a radial direc- 



ILT~- 



~~~ec 



~ms 



ec- 



ertr 



-71' 



Fn;. 158. — Stage of Clon'li ,m Itisxnuiia showing the f'oriiiation of the neural tube 

 (after VAN Beneden and Julin). A, dorsal view ; the cell-boundaries drawn are 

 those between the cells of the dorsal entoderm-wall ; B, median sagittal section. 

 ch, rudiment of the chorda; ec, ectoderm; en, entoderm ; ?ns, mesoderm : n, cells of 

 the neural plate; n', roof of the medullary tube; nr, medullary tube; n/>, the 

 neuropore, still very large. 



tion. the entoderm thus becoming multilaminar 

 in reality reached through epibole (Fig. 

 156). In the anterior region of the body 

 this overgrowth is especially marked, while in 

 the posterior half, a small pit-like depression 

 (Fig. 156 B) indicates the last remains of an 

 invagination-cavity. This cavity, however, 

 completely disappears after the blastopore has 

 closed. The entoderm then represents a solid 

 mass, the cells of which are soon found to 

 vary in size. The position of the blastopore 

 is occupied by the neural plate (»). 



The gastrula-stage is here 



ch 



During these stages, the medullary 

 plate, which is already somewhat invagin- 

 ated, changes into a closed medullary 

 tube, its lateral walls, the medullary 

 folds, growing towards one another and 

 fusing (Fig. 160). 



■ee 

 Fig. 159. — Transverse section 

 through an embryo of Clave- 

 lina (after van Beneden and 

 Julin* ). ch, rudiment of the 

 chorda : ec, ectoderm ; en, ento- 

 derm ; ins, mesoderm-diverti- 

 culum ; in"', medullary told- ; 

 », medullary plate. 



The medullary tube develops from behind forward, a special part being 

 taken in the process by the fold which connects the two medullary swellings 



