Embryology cfcc. of Yoldia limatula, Say. 269 



From a time preceding the formation of the apical plate a 

 few cells of ectodermal origin have occupied an anterior 

 position. These cells extend ventrally to the depression x 

 (fig. 1), and push in between the test-cells. It was appa- 

 rently from cells connected with this group that the apical 

 plate was formed, and now, at about forty hours, other cells 

 from this same group form two thick-walled pouches, which 

 open to the exterior between the test-cells. These are the 

 rudiments of the cerebral ganglia. Fig. 2 represents a trans- 

 verse section of an embryo of fifty-eight hours, taken through 



Fig. 2. 



Transverse section of an embryo of fifty-eiglit hours, taken through the 

 depression x, fig. 1. eg, pouches which form the cerebral 

 gangha ; ing, wall of the anterior end of the mid-gut ; t, test. 



the depression a: (fig. 1). The test, t, is shown to be com- 

 posed of large vacuolated cells, the walls between which are 

 no longer very distinct. As this section cuts the anterior 

 row of test-cells, they are uniformly covered with cilia. The 

 rudiments of the cerebral ganglia, cg^ appear as two thick- 

 walled pouches. The wall of the anterior end of the mid-gut, 

 rng, appears in section. 



Fig. 3 represents a transverse section of another embryo at 

 a corresponding stage, taken just in front of the second band 

 of cilia. The shell-gland, sg, has become arched upward, 

 and is on the verge of protruding laterally to form the 



