490 L. S. STONE 



line with the results obtained in the removal of placodes in the 

 trigeminal region and shows that the general cutaneous system 

 is placodal in nature and not, as Landacre ('10) has suggested, of 

 neural-crest origin. 



From the results obtained in the series of experiments recorded 

 in this paper there can be no doubt that the lateral-line sensory 

 system in Amblystoma is derived entirely from placodes. The 

 study of the preauditory placode shows that a large part of the 

 lateral-line ganglion of VII is formed from this placode and from 

 its anterior end arises the supra-orbital primordium of sense 

 organs. The other lateral-line primordia in the head region are 

 separate in origin as in Necturus and also contribute to the VII 

 lateral-line ganglion. At no time is there any condition such as 

 that described in Lepidosteus by Landacre and Conger ('13) in 

 which the preauditory placode begins to disintegrate at the time 

 when the first trace of the lateral-line primordium can be detected. 

 It is quite possible that Landacre and Conger were misled in this 

 interpretation of the preauditory placode, for, according to their 

 description, it apparently arises very early, and although they 

 describe no cells being given off from the placode, it seems prob- 

 able that there may have been an early contribution which was 

 unobserved. 



In the case of the postauditory lateral-line primordia, the 

 study of experimental as well as of normal material shows that 

 the three trunk lines of sense organs have separate primordia, 

 and in this respect Amblystoma is similar to Necturus. The 

 experimental results show that the occipital group of sense organs 

 appeared in a few cases where the ectoderm in the anterodorsal 

 portion of the gill swelling (fig. 61) was not entirely removed, al- 

 though the ear was entirely removed. This condition implies 

 the independence of the occipital primordia from the auditory 

 placode and also indicates in embryos close to stage 21 the ability 

 of the ectoderm in the anterodorsal region of the gill swelling to 

 give rise to occipital primordia. The complete removal of the 

 postauditory lateral-line primordia was not only accompanied 

 by the absence of the groups of sense organs, but by an entire 

 absence of the lateral-line ganglia. When only a few sense 



