THE PYCNOQONIDS. 7 



rounding cells are of mesoblastic origin. In Pallene, as we shall 

 see later, there are similar cavities, which are undoubtedly meso- 

 blastic in origin. The mid-gut ends blindly behind, as is seen in 

 other sections. 



In Fig. 14 is shown part of a section through a more anterior 

 part of the body. The section passes through the brain above 

 and the first pair of ganglia below. These two are connected by 

 the circumcesophageal ring. This commissure is composed of 

 both cells and fibers. In the next section to the one figured a 

 broad band of nerve fibers passes on each side from the brain to 

 the ventral ganglia, and in Fig. 14 a few of these fibers may also 

 be seen. In the middle of the commissure lies the cross-section* 

 of the oesophagus. Its lumen is triangular and its walls com- 

 posed of a layer of cells, which are clear around the periphery of 

 the triangle. Around the oesophagus are a few scattered meso- 

 blast cells. From the brain, B, there is part of its substance 

 which projects ventrally towards the oesophagus and is quite 

 conspicuous in sections of the Pantopod-larva in this region. On 

 each side of the brain are seen a pair of diverticula of the mid- 

 gut. These go to the first pair of appendages and are traceable 

 to D 1 of Fig. 13. The first pair of appendages are, as Dohrn has 

 shown, innervated from the brain. 



The section of the embryo following Fig. 13 gives a cross-sec- 

 tion of the pair of simple eyes. A part of this section is shown 

 by Fig. 15. In the middle line of this section above the posterior 

 part of the brain, B, the ectoblast is thickened beneath the 

 cuticle, and immediately on each side of the middle line appear 

 the pair of eyes. Each is seen in section to be composed of 

 three cells — two clear outer ones with large nuclei and an inner 

 much pigmented cell. Around the eyes the ectoblast is quite 

 thick, and on each side it sinks down slightly from the surface, 

 suggesting that the eye may be here increased in size. 



In Fig. 16 is a section through the third pair of ventral ganglia 

 which we saw on the ventral side of the embryo posterior to the 

 fused pairs of first and second ganglia. The section shows that 

 at this place the epiblast is greatly thickened and the cells 

 columnar, with clear outer portions. In the ganglion on the left 

 of the section the cuticle seemed to dip down into the center of 

 the ganglion, where it becomes extremely thin. This was not 



