EMBRYOLOGY. 



713 



Fly. -'•-,); the limestone rods, simply T-shaped before {Fig. 40), are sending 

 off small processes ; and the chords of vibratile cilia (v, r'). which were a 

 simple button [v, Fig. 40), are quite stout, projecting beyond the general 

 outline when seen from above or below {Fig. 41, v). It is at this advanced 

 condition only that the oesophagus touches the lower surface, previous to the 

 formation of the mouth, which takes place only when the embryo has reached 

 the condition of Fig. 44. A view of Fig. 41, seen from the anal extremity 

 {Fig. 42), shows how far it has lost its cylindrical shape and become wedge- 



FlO. !50. 



Fig. 40. 



Fig. 41. 



shaped. In Fig. 43, which is the same pluteus seen in profile, the indentation 

 which indicates the position of the mouth (in) lias changed somewhat the 

 even outline of the lower surface ; there is a marked bending of the alimen- 

 tary canal, bringing the anal opening ( :) still nearer the lower surface. In 

 fact, since the first formation of an opening in Fig. 19 {a, m), which is at once 

 strictly mouth and anus, there is a continued tendency in the anal part of 

 the digestive cavity to bend towards the oral surface, even while this single 



Fig. 4". ' .• 



opening performs the functions of mouth and anus, during the period which 

 precedes the formation of the mouth, after the alimentary canal, the true 

 stomach, and the oesophagus have been differentiated. This is somewhat 



