ON THE DEVELOPMENT OF LINGULA ANATINA. 19 



The description of the structure of the gastrula having been 

 given, we may, for convenience's sake, next follow the external 

 developmental changes up to the oldest embryo that I was able 

 to rear from the egg. Then in the section following I shall enter 

 into the more minute structural characters of the embryos during 

 these stages. 



a. External fokm. 



In Fig. 34. (PL III.) an embryo a little more advanced 

 than the gastrula is represented in side view. At the upper pole 

 the blastopore is closed and at this place the mes-entoblastic 

 cell-mass is connected with the ectoblast. This pole becomes the 

 anterior face and the opposite end, which is lowest in the figure, 

 the posterior face of the embryo of the ensuing stages. As the 

 embryo at this stage is quite spherical or a little compressed 

 antero-posteriorly, we cannot as yet determine which side re- 

 presents the future dorsal or ventral side. With the growth of 

 the embryo, the ectoblast begins to fold outward horizontally in 

 all directions to form the ring-like mantle fold, until the embryo 

 assumes nearly a disc-shape, flattened antero-posteriorly. An 

 embryo at this stage is represented in two views ; the one (PI. 

 III., Fig. 35.) from the anterior face, the other (PI. III., Fig. 

 36.) in an aspect which is either ventral or dorsal. In the 

 former figure we see the central area (am. rg.) elevated from 

 the general surface into a hillock. This area I shall call the 

 arm-ridge, for it subsequently gives rise to the arm-apparatus. 

 The surface of the ridge has a warty appearance owing to the 

 fact that the cells composing it are still large and have their 

 ends greatly bulged outwards. Surrounding the arm-ridge there 



