THE DEVELOPMENT OF THE EARTH-WORM. 241 



little less distinct ; no other fibres are seen, unless faint traces 

 of prolongations of the adjacent cells, visible with a higher 

 power, are regarded as such. These are the rudiments of 

 the fibrous commissures. Fig. 26 d shows the plate again 

 in the form which it had further back, but the granular sub- 

 stance of the commissure, which encloses some nuclei, is 

 still more conspicuous than in the preceding section, and the 

 two lateral rudiments are fused in the median line and form 

 the bottom of the dorsal furrow. The sides and the ventral 

 portion consist of a continuous pretty thick layer of cells. 



The same succession of such alternate zones repeats itself 

 again several times in the backward direction, then every 

 trace of the commissure is lost, and tho medullary plate passes 

 by every gradation to the state of fig. 25. In front similar 

 conditions are observed ; here, however, every section shows 

 the presence of a commissure in a stage of very much more 

 perfect development. 



To illustrate the subsequent changes I select a group 

 of sections of an embryo of 4*5 mm. in length (PI. XI, fig. 

 27 a, by c, d, e). In the first preparation (27 a) the medullary 

 plate has a shallow impression both on the ventral and dorsal 

 surface. The cells occupy the surface, leaving the median part 

 of the upper side free, and are especially accumulated in the 

 lateral processes; from here they are continued round to 

 the inferior surface, where they unite and penetrate deeply 

 into the interior of the plate, so that this again appears to 

 be divided into two lateral cords, whose centres are composed 

 of the granular substance. Immediately in front (fig. 27 b) 

 the plate becomes kidney-shaped. The septum, which pro- 

 jects from the cortical layer of cells into the interior, is much 

 more developed, and divides the commissure nearly com- 

 pletely into two trunks. But in the same section the firm 

 union of the cells is relaxed and they separate a little to 

 the right and left, occasioning the appearance of a kind of 

 vertical fissure, which is more distinct in the centre than at 

 the periphery. This is clearer still in fig. 27c. The cellular 

 process penetrates a little less deeply into the substance of 

 the commissure, but the fissure which divides it into two 

 parts is more evident, especially at the centre, where it ends 

 in an enlargement. Further, the whole cellular covering is 

 thickened considerably, diminishing the size of the commis- 

 sure. But in the following section (fig. 27 d) that constric- 

 tion has entirely disappeared ; the commissure which forms 

 a large mass containing some scattered nuclei, is surrounded 

 by a uniform layer of cells. The median thickening of the 

 cellular covering reappears once more in the section (fig.27 e)^ 



