344 



TUNICATA. 



blastopore and representing the boundary between the ectoderm and 



the entoderm (Fig. 154, x). 



Tn these later gastrula-stages the commencement of histological 



differentiation is already evident. 

 This does not consist merely in 

 the distinction between the ecto- 

 dermal and the entodermal elements, 

 although the latter are larger, more 

 strongly granular and darker in 

 colour; but differentiations are 

 already to be found within these 

 germ-layers. The ectoderm-cells 

 which bound the blastopore, for 

 instance (Fig. 154 A, n), are dis- 

 tinguished by the large size of their 

 nuclei, their greater affinity for 

 carmine stain, and their cubical 



shape from the other ectoderm-cells, which soon become flattened. 



This ring of cells is the first rudiment of the central nervou* .sv/x/V///, 



and, as the blastopore closes more and more, changes into the 



FIG. 153. Dorsal aspect of an embryo 

 of Clavelina (after SEKLIGER). b, b', 

 b", outlines of the blastopore at three 

 consecutive stages of development. 



FIG. 154. Gastrula-stage of Clavelina Rissoana (after VAN BENEDEN and JULIN). 

 A, dorsal aspect; B, median sagittal section, b, blastopore; ec, ectoderm; en. 

 entoderm ; n, cells ol the nerve-ring ; x, small, wedge-shaped cells. 



