88 CASWELL GRAVE 



anterior end, with a comparatively large cone-shaped mass of 

 yolk material, the yolk granules and masses seeming to be included 

 within enormously developed endodermal cells (figs. 1 and 2). 

 The lateral walls of the posterior portion of the pharynx are in 

 contact with the inner walls of the right and left horns of the 

 atrium and a series of open communications between the pharyn- 

 geal and atrial cavities has been formed by the differentiation of 

 three horizontal rows of ciliated gill openings; the first or dorsal 

 row consisting of seven small openings, the middle row of six 

 somewhat larger ones, and the ventral row of five comparatively 

 large gill openings (figs. 1 and 3). 



The part of the alimentary canal in which oesophagus, stomach, 

 and intestine are differentiated communicates with the pharynx 

 at a point near the posterior end of the yolk mass and slightly 

 to the right of the longitudinal axis of the body. The stomach and 

 intestine have the form of a horizontal loOp in the midventral 

 region of the body beneath the yolk mass. The rectal portion 

 of the intestine pierces the left horn of the atrium and the anus is 

 found near the base of the atrial siphon (fig. 1). It is perhaps 

 needless to state that the alimentary tract is not functional 

 during the larval period. 



NOTOCORD AND ENDODERMAL STRAND 



The notocord occupies the proximal two-thirds of the axis of 

 the tail and penetrates the body to a point directly below the 

 middle portion of the sensory vesicle, where it ends in contact 

 with a thickened portion of the pharynx just behind the pointed 

 posterior end of the yolk mass (figs. A, C, and 1). It retains no 

 trace of its cellular origin, but at this stage is made up of a thin 

 cortical sheath of dense material in which granules and larger 

 spherical bodies are embedded, and a central medullary core 

 composed of non-staining substance, at the periphery of which 

 a tracery of delicate strands can be made out (figs. 3, 8, and 10). 



A longitudinal linear series of cells, known as the 'endodermal 

 strand,' occupies a portion of the space on the right side of the 

 notocord between the overhanging edges of the muscle bands. 

 One or sometimes two 'strand' cells can usually be made out in 



