MUSEUM OF COMPARATIVE ZOOLOGY. 



179 



Twenty-four hours later, about the eighth day, the pulsating sac of the 

 foot has become still larger, and the oral sinus has extended backward and 

 downward as a very narrow tubular passage, — the oesophagus, — which 

 follows the surface of the nutritive sac for some distance, and subse- 

 quently opens into it. The peculiar ciliated cells of great size and spongy 

 appearance, which occupy a linear tract along the middle of the roof of 

 the mouth and oesophagus, are at this time very prominent (Plate I. 

 Fig. 2, loph. cil). These cells form what Fol ('80, pp. 190, 191) has 

 called the "ciliated ridge." They persist until after the completion of 

 the nervous system. The ingrowth of the ectoderm to form the rectum 

 is now composed of a compact group of small cells, which shows a small 

 lumen in its central portion, but is still closed at both ends. 



The cei'ebral ganglia remain in neai'ly the same condition as that last 

 described. About twelve hours later, between the eighth and ninth days, 



Figure A. — The right face of a section parallel to the sagittal plane from an embryo of the 



nUith day X 220. 

 gn. pd. Pedal ganglion o cy. s. Left otocyst. 



the two cerebi-al invaginations have become deeper, and the two groups 

 of cells which form the main portions of the corresponding ganglia con- 

 tain a greater number of cells. (Plate II. Fig. 15.) 



The pedal ganglia are also now composed of many more cells than in 

 the previous stage. Each ganglion is usually pear-shaped, and tapers to- 

 wards the posterior end of the foot. They both continue to receive ac- 

 cessions from the ectoderm (Figure A), and at the same time are rapidly 

 increasing in size by division of the cells already in position. The nuclei 

 are larger and more easily distinguished than in the previous stage from 

 those of the mesodermic cells, the latter being more spindle-shaped than 



