No. 3.] THE CELL-LINEAGE OF NEREIS. 421 



retain their original arrangement {cf. Fig. 86). They are every- 

 where closely surrounded by the ectoblast except at the sides, 

 where the mesoblast-bands intervene (Fig. 81). The prototroch 

 encircles the equator of the embryo ; the mouth lies in the 

 median ventral line about half-way between the prototroch and 

 the margin of the pigment-area. There is no post-oral band of 

 cilia, and no ventral ciliated area. The prototroch shows the usual 

 interruption in the median dorsal line. On the upper hemisphere 

 a pair of eye-spots have appeared, the position of which may be 

 seen by a comparison of Figs. 82, 85, %6. On the anterior half 

 of the upper hemisphere are five spherical bodies arranged in a 

 regular arc, one of them lying in the median line, the others 

 symmetrically placed on either side of it. These bodies, which 

 I at first mistook for sense-organs, I shall call the frontal bodies. 

 Each of them appears to be developed out of a single cell, in 

 which appears a clear space like a vacuole surrounded by a 

 layer of granular protoplasm. The clear space stains intensely 

 with hasmatoxylin precisely like the contents of the gland-cells 

 that occur so commonlv in the larvae of other annelids (e.G". in 

 Spirorbis or Terebelld) ; and from this fact and from their later 

 history I am led to regard them as glands. It will be shown 

 further on that these bodies are of great importance in the 

 orientation of the larva, since tJiey mark the anterior extremity. 



The ventral plate (Fig. 81) is composed of crowded prismatic 

 cells. Posteriorly it ends in two distinct groups of cells (JT, 

 Figs. 81, 82), which I believe to be the last traces of the residual 

 teloblasts. These groups of cells are separated from the proto- 

 troch on each side by a lateral area and from each other by a 

 median triangular area, over which the cells are very thin and 

 delicate and show a marked contrast to those of the ventral 

 plate. The triangular area represents the median portion of 

 the dorsal region, the lateral areas the lateral portions of the 

 dorsal region of the adult body. The relations of these regions 

 to the cleavage-blastomeres have been already given. 



There is still no sign of metamerism or of a body-cavity. On 

 the upper hemisphere the ectoblast shows distinctly the begin- 

 ning of the supra-oesophageal ganglia in the form of a broad 

 transverse thickening of the ectoblast (neural plate), narrower 

 in the middle, and extending down on either side to the proto- 

 troch. The central part bears the apical tuft of cilia, still in 



