ON THE DEVELOPMENT OF LINGULA ANATINA. 29 



It should here be noted that in the early stages both the 

 ecto- and ento-blast cells in the part facing the segmentation 

 cavity retain their original character in having yolk-granules as 

 in the blastula. This is best shown in Fig. 59 (PI. IV.) which 

 is a portion of the ectoblast from the same section represented 

 in Fig. 58 (PI. IV.). 



2. Mesoblast, Body Cavity and Muscles. 



As has already been pointed out in the study of surface 

 views, the mesoblast cells are proliferated from the lateral walls 

 of the entoblast, and form with the latter a compact mesento- 

 blastic cell mass. Afterward the mesoblast cells group themselves 

 in two masses closely apposed to, but with a distinct line of 

 demarkation from the alimentary canal (PI. IV., Fig. 62, 63, 

 65, 68. msb.). As the masses are placed on the right and left 

 of the alimentary canal, the latter comes in direct contact with 

 the ectoblast on both the dorsal and ventral surfaces. It should 

 be stated that Shipley ('83 p. 511) observed the alimentary 

 canal laid against the dorsal ectoblast in the embryo of Gistella. 

 The mesoblast cell mass soon hollows out and the cells are 

 flattened, coming gradually to line the entoblast on one side and 

 the ectoblast on the other. The lumen thus formed is the body 

 cavity (PI. IV., Figs. 63, 65. el.). In Lingula, therefore, the 

 body cavity arises after the schizocoelic type. 



Anteriorly a part of the mesoblast cells becomes loose and 

 mesenchymatous (PL IV., Figs. 58, 60, 61, 62, 65, 66, 61, 69. 

 ms.). These cells find their way into the arm-apparatus, and 

 finally fill the arm-sinus loosely, leaving much space between 

 cells (PL IV., Fig. 69. ms.). Even after the mesenchyme cells 



