384 E. RAY LANKESTER. 



of which are not large and granular, but scarcely distinguish- 

 able), the pedicle of invagination. 



The sac composed of large cells ver}' early becomes con- 

 stricted, so as to present two lobes, as seen in PI. XVII, 

 fig. 1. In looking at the figures in PI. XVII it must be 

 remembered that the specimens are often compressed, and 

 that, only an optical section or partial view can be given of 

 the various parts ; hence the mass of large cells (the gas- 

 triila-eiidoderm) is frequently distorted. The lobes appear 

 at first to lie right and left^ the pedicle being in the median 

 plane. 



The pharynx now commences to develop with the in- 

 pushing of the mouth from the body-wall, and gradually 

 extends downwards into the mass of endoderm-cells, so as to 

 be partly concealed by them (figs. 8, 11, 17 j»/?). At the 

 same time the cells in the pedicle of invagination differentiate. 

 The pedicle assumes a tubular character, and its parietal end 

 becomes bent round, so that the tube terminates as a shortly 

 reflected csecum. Whilst the pharynx and the intestinal 

 portion of the alimentary canal are thus differentiating, 

 changes have been going on in the gastrula-endoderm- 

 cells, to which changes I have already alluded. In place 

 of a bilobed group of large granular cells we now have a net- 

 work of fine granular filaments with nuclei at intervals 

 completely enclosing and surrounding on all sides pellucid, 

 highly refracting spheres (fig. 22). Moreover, a tunic of 

 fusiform cells, of the same character as the elementary mus- 

 cular cells which are seen in other parts of the embryo, has 

 spread itself over the whole of the alimentary tract (fig. 

 IT, tge). They are closely fitted to the pharynx and rectum 

 (figs. 21, 22), and also extend over the pellucid spheres and 

 their granular network, whence they send branches to the 

 similar cells lining the body-wall (figs. 17, 22). Whence has 

 this tunic developed ? At the pharyngeal end the cells are 

 clearly continuous with those of the body-w*all, and at the 

 rectal end also ; but those enclosing what was the gastrula- 

 endoderm are probably developed from the processes which the 

 invaginate gastrula-cells send to the body-wall, even in the 

 trochosphere stage of development, as seen in PI. XVI, 

 fio-. 14. If this is the case the musculature of the terminal 

 portions of the alimentary canal will have been developed, 

 like the musculature of the body-wall, from the ectoderm of 

 the gastrula, whilst the musculature of the middle portion 

 of the alimentary canal and its appendices will have been 

 developed from the gastrula's endoderm. 



We have, however, yet to see what eventually comes of 



