302 A. T. MASTERMAN. 



layer of the mesoblastic lining of the collar cavities. The 

 course of the oesophagus is first upwards towards the dorsal 

 surface, and then curved backwards (PL 20, fig. 20; PI. 21, fig. 

 38), and has a straight direction till it opens into the dorsal 

 anterior end of the " stomach," which is partially differentiated 

 into a pharynx. 



The part of the "stomach" lying in the collar region is 

 separated by a constriction from the part behind, and in fact in 

 some cases the constriction forms a short tube running from the 

 anterior part or pharynx to the posterior part or true stomach. 



The pharynx has small cubical cells with regularly disposed 

 nuclei; the protoplasm stains homogeneously. They have 

 small cilia, but the contrast of these cells with the " oeso- 

 phageal" cells is marked and the transition abrupt (cf. PI. 19, 

 figs. 9 and 10). 



The cells situated in the mid-dorsal region of the pharynx 

 are, however, for some way columnar and strongly ciliated 

 (fig. 16), and altogether resemble more the "oesophageal" 

 type. In the antero-lateral region the pharyngeal wall is pro- 

 duced into two remarkable diverticula, which in the fully 

 developed larva lie, as a pair of elongated organs (notochords) , 

 laterally to the oesophagus (figs. 9, 22, nch.). 



The cells are columnar, with homogeneous protoplasm, but 

 in the course of development they undergo a remarkable 

 transformation. The earliest stage yet found by myself (PI. 21, 

 fig. 30) presents the diverticulum as a hemispherical protube- 

 rance. Sections at the level of a, b, and c exhibit the characters 

 shown in figs. 30«, 306, and 30c. 



In fig. 30fl! is seen a section through spherical vacuoles with 

 a few nuclei squeezed in between them. The vacuoles are 

 filled with a clear, homogeneous, non-staining substance. (I 

 have failed to stain them in the slightest degree with hsema- 

 toxylin, carmine, picric acid, iodine, and numerous aniline 

 stains, such as eosiu, methyl green, safraniu, &c.) 



In fig. 30c the central cavity is reached, and the cells are 

 seen in longitudinal section. This reveals that the vacuoles 

 are regularly disposed at the distal extremity of each cell, one 



