236 NIKOLAS KLEINENBERG. 



in front of the oral margin (Scheitelplatte). Soon two 

 filamentous thickenings of the ectoderm begin to extend 

 themselves from the lateral regions of the Scheitelplatte 

 backwards along the sides of the mouth into the neighbour- 

 ing segments, where thev lie on either side of the middle 

 line."i 



I should agree with this as regards the fact that a dorsal 

 plate arises before any other part of the central apparatus if 

 I were capable of forming a clear idea of what the author 

 intends to express by these words, and if I were convinced 

 tliat he really has observed the first stages. But the 

 assertion that the ventral medulla is produced from two 

 prolongations of the cephalic ganglion I believe to be 

 entirely erroneous if the probability be admitted that, in two 

 species of the same genus, the principal organs would be 

 formed in the same way. 



A few words on the further transformations of the dorsal 

 medullary plate. The whole rudiment separates at once 

 from the ectoderm, and becomes enveloped in a sheath of the 

 somatic lamina. From the anterior median part of the arch 

 start two prolongations, which enter the upper lip, where 

 they appear to become confounded again with the ectoderm, 

 which here is transformed into sensitive epithelium. In 

 like manner, the opposite side of the rudiment sends out 

 processes directed backward, -which are broader and longer 

 than the anterior. Thus, the cephalic ganglion seen from 

 above appears to consist of two pear-shaped halves broadly 

 joined in the middle. The two lateral projections which 

 form the dilated extremities of the arch, also separated from 

 the ectoderm, extend gradually as much upwards as down- 

 wards, and unite principally with the median arch of the 

 medullary plate. In transverse sections this is seen to 

 embrace already more than half the oesophagus. In the 

 median dorsal line is seen a deep impression where the 

 dorsal blood-vessel is placed ; the margins of this groove rise 

 a little, and these bondings descend nearly vertically towards 

 the ventral surface, where they end in very fine extremities 

 ■without joining the ventral chain. I do not wish here to 

 enter into the description of histological differentiations; I 

 will only say that the transverse commissure which connects 

 the two halves of the arch appears in this stage, and is the 

 first to arise. All the cells on the ventral face become trans- 

 formed into a finely granular substance, while at the sides 



^ " BeitragezurEntwicklungsgeschichte und Morphologic der Anneliden," 

 ' Sitzuugsberichte der Akadernie der Wissenschafteu iu Wieu,' T. Ixsip, 

 1S76, p. 1. 



