THE DEVELOPMENT OF THE EARTH-WORM. 225 



opposite side; a semicircular commissure is thus formed, 

 situated on the back between the mouth and the cells, which 

 unite the two embryos. Figs. 16 and 17 represent sections 

 of the head end, in which the formation of the commissure of 

 the streaks is ah-eady completed, and its position relatively 

 to the surrounding parts is easily recognisable. Fig. 16 5 is a 

 ifsection immediately behind fig. 16 a, and serves to show the 

 continuity of the cephalic arch or commissure with the cords 

 which occupy the lateral parts of the body. But if in the 

 first stages the enlargement of the streaks is owing chiefly to 

 the junction of cells derived from the ectoderm with the 

 mesoderm, this holds good, above all for the formation of the 

 commissure. It is certain that only a very few cells pre- 

 formed in the mesoderm enter into this ; the larger part are 

 derived directly from the ectoderm, which thickens, until 

 three or four layers of superimposed cells appear (fig. 22 pc), 

 the deepest of which then separate themselves from the more 

 superficial to become blended with the mesoderm of the 

 lateral germinal streaks. 



Rathke speaks of the origin of the cephalic portion of the 

 germinal streaks in JSephelis and Clepsme in such vague 

 terms that it cannot be clearly understood,^ and Kowalewsky 

 does not make any explicit statements on this question, but 

 he figures an embryo of Euaxes', in which the union of the 

 germinal streaks on the back is perfectly clear. Lastly, 

 C. Semper, after having found a special germinal streak for 

 the formation of the head in the reproduction by fission and 

 gemmation of the Naidse, describes also in Clepsine the 

 origin of the cephalic streak from two lateral thickenings, 

 which are at first independent of each other and of the 

 ventral germinal streaks, and therewith strengthens his 

 theory of the original distinction between head and trunk. 

 In o])position to this I affirm that in Lumbricus trapezoides 

 there is never a special rudiment for the preoral ring, but 

 that the cephalic lobe, whose subsequent changes are so 

 important, is formed simply by the union of the germinal 

 streaks on the back. 



This dorsal commissure, which I shall henceforward call 

 the cephalic germinal streak, becoming greatly thickened, 

 raises itself above the mouth in the shape of a semilunar 

 fold or incomplete ring. After this the entrance to the 

 digestive cavity, which till now was a small fissure some- 

 times very difficult to recognise, becomes transformed into a 

 semicircular fossa, deep at the dorsal side, where it is 

 surrounded by the projecting cephalic germinal streak, and 

 ' Loc. cit., pp. 29, 95., 



