THE DEVELOPMENT OF THE EARTH-WORM. 2S5 



cannot have much authority. They are open to the greatest 

 variety of interpretations and objections, but it is a little too 

 much when Semper, taking advantage of an easily explicable 

 inexactness of expression in Leuckart's notice, twists it in an 

 extravagant manner to make it fit his own observations and 

 speculations. In fact, the mode of formation of the oesopha- 

 geal collar, which Semper thinks typical for all the Annu- 

 lata, is not consistent either with the observations of Rathke 

 and Leuckart on the Hirudinea, nor with my own on 

 Lumbricus. 



During the gemmation of the Naidse, according to the 

 above-quoted observer, the ventral germinal streak in the 

 cephalic zone splits into two parts, which grow up on the 

 lateral walls of the oesophagus, arching over towards one 

 another on the dorsal surface. As soon as they have em- 

 braced the intestine a portion of them separates itself to form 

 the commissure and the ganglionic substance of the brain ; 

 the two halves of the supra-oesophageal ganglion thus 

 formed then fuse to one another in the median dorsal line. 

 This portion of the oesopbageal collar is derived from the 

 mesoderm. But then the ectoderm developes to the right 

 and left a kind of bud, which Semper calls " Sinnesplatte," 

 because in it is formed the eye of the Naidac, which is 

 directed towards the dorsal surface, where it enters into the 

 composition of the supra-oesophageal ganglion. Hence the 

 entire oesophageal collar would be a product of the ventral 

 germinal streak, together with two lateral buds of the ecto- 

 derm, without the intervention of a dorsal medullary plate, 

 and thus would be an organ heterogeneous, even in its essential 

 parts, being derived as much from the mesoderm as from the 

 ectoderm.^ I have no observations of my own on the 

 development of this organ in the agamic generation of the 

 Naidse, but I know that in a group of animals, closely related 

 to these, the embryonic development proceeds in quite a 

 different manner. In Lumbricus the first rudiment of the 

 oesophageal collar is a dorsal medullary plate, which arises 

 independently of the ventral chain and exclusively from the 

 ectoderm. I do not know what forms the sensitive plates of 

 Semper, since it does not appear justifiable to identify them 

 with the terminal enlargements of the medullary plate. 



Lastly, Hatschek, in opposition to Semper, describes the 

 aff"air very differently. He says : " The first rudiment of 

 the nervous system is found in Lumbricus in those embryos 

 in which the foremost segments are developing the seg- 

 mental organs. It appears as a thickening of the ectoderm 



' Loc. cit., pp. 206, 210, and elsewhere. 



