ASCIDIACEA FORMATION OF THE GERM-LAYERS. 



347 



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

 retain their uuilaminar arrangement, but become distributed in a radial direc- 



ec- 



"xi 



JIT 



Fi<;. 158. Stage of ClaveUita Iii.w>fna showing the formation 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* 

 rli , rudiment of the chorda ; ec, ectoderm ; en, entoderm ; ms, mesoderm ; n, cells of 

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

 neuropore, still very large. 



tion, the entoderm thus becoming multilaminar. The gastrula-stage is here 



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 (n). 



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). 



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 ; ms, mesoderm-diverti- 

 culum ; mw, medullary folds ; 

 n, 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 





