THE DEVELOPMENT OF THE EARTH-WORM. 233 



a new organ, the " cephalic medullary plate," has become 

 separated from the peripheral ectoderm^ which once more 

 forms a unicellular covering. Fig.-^ 5 shows the same 

 arrangement nearly unaltered ; but in the third section 

 (^g.^20 c), instead of a continuous semicircular thickening, 

 there are two large projections of ectoderm (gc), which 

 thrust themselves into the cephalic cavity, separated from 

 each other by a largish tract of simple ectoderm. 



These projections are still more conspicious in ^g.2&d. ■' :' 

 At the ventral side are seen in the last section two eleva- 

 tions, formed of small cells very like those of the rudiment 

 of the cephalic ganglion, and separated from each other by a 

 furrow, whose floor is formed of ciliated cells (m). This is 

 the section of the rudiment of the first ganglion of the ven- 

 tral chain. It is important to notice that there is no con- 

 nection between this and the dorsal thickening {gc). In 

 the three following sections the last are still recognisable 

 although much reduced ; further back they are altogether 

 absent. Fig. 21 a, b, c, c?, are longitudinal horizontal sec- 

 tions of an embryo 0'6 mm. in length ; 21 a is the fifth of 

 the series, going from the ventral to the dorsal surfaces. It 

 is to be understood that when the embyro is placed horizon- 

 tally the first sections pass through the very prominent belly 

 without touching the mouth or head end. The section is 

 not perfectly at right angles to the vertical axis, but has 

 fallen with its left side nearer the ventral surface than the 

 right, hence the difference. On the left side the ectoderm 

 appears thickened, and this is the section of the longitudinal 

 ventral chain of ganglia (n) ; on the right side and in front 

 the ectoderm consists of a single layer of pavement-cells 

 (ec). In the segment which comes next (21 h) the ectoderm f<2^c^''*^^ 

 cells on the apex of the head have become long and cylin- 

 drical, but are still placed in a single layer. A little further 

 back the ectoderm shows on each side a spindle-shaped 

 swelling ((/c), which loses itself again in the unicellular 

 layer covering the body. The same conditions of the ecto- 

 derm are seen likewise in the seventh and eighth sections, 

 in which the lateral thickenings are still larger. But in the 

 ninth (fig. 21 «?) the cylindrical epithelium which separated 

 the swellings in front has disappeared, and these are united 

 by a largish commissure ; they form together an arch em- 

 bracing the cephalic extremity. Finally, in the tenth sec- 

 tion (fig. 21.) no further trace of the lateral thickenings is 

 found ; the ectoderm is, on the contrary, much thickened in 

 the middle line. 



Now, the comparative combination of these sections will be 



