344 



TUNIC ATA. 



blastopore and representing the boundary between the ectoderm and 

 the entoderm (Fig. 154, ./■). 



In 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 nervous system, 

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



Fig. 153. — Dorsal aspect of an embryo 

 of Clavelina (after Sebliger). b, V, 

 b", outlines of the blastopore at three 

 consecutive stages of development. 



Fig. 154.— Gastrula-stage oi Clavelina Rissoaiux (after van Beneden and Junxi 

 .1. dorsal aspect; /,', medial: sagittal section, b, blastopore; ec, ectoderm- en 

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



