CRANIAL GANGLIA OF AMBLYSTOMA 489 



it shows that an extensive disturbance of the crest cells as early 

 as stages 21 and 23 does not inhibit the growth of the ganglion. 



As Coghill ('16) has already shown in Amblystoma, there is an 

 early contact of the gasserian ganglion with the ectoderm. At the 

 point of contact near the anterior border of the preauditory plac- 

 ode (figs. 21 and 26) is a small thickening in the ectoderm which 

 can be followed only through stages 28 to 30. Although this 

 condition has not been described in any other forms, it is quite 

 possible that it does exist, but has been overlooked on account of 

 its small size and short duration in a region where the crest cells 

 are very abundant. Among the cases where the removal of the 

 preauditory placode included ectoderm near the posterodorsal 

 border of the optic vesicle there occurred two cases (fig. 56 b) in 

 which there was no gasserian ganglion. When smaller areas of 

 ectoderm were removed from the posterodorsal region of the 

 eye there often occurred small gasserian ganglia. This was pos- 

 sibly due to the fact that not all of the placode had been removed. 

 In one case where there was a deficiency in the crest cells on the 

 mandibular arch after the crest cells had been removed a smaller 

 gasserian ganglion was observed. In this case it seems quite 

 possible in the light of the control and other operations that the 

 gasserian placode was injured when the ectoderm was reflected 

 at the time of operation. 



The remaining portion of the general cutaneous system of the 

 cranial nerves is to be found in the X. In the observations re- 

 ported in this paper no definite distinction could be made in the 

 early stages between the small general cutaneous and the visceral 

 ganglia of X. Coghill ('16) has observed that during its early 

 contact with the ectoderm, the cutaneous ganglion of X has no 

 connection with the brain. However, when a large area of ecto- 

 derm was removed containing the epibranchial placodal regions of 

 IX and X no definite general cutaneous fibers could be found. 

 This leads one to conclude that the general cutaneous portion of 

 the vagus complex is derived from the lateral ectoderm and the 

 early contact of the small general cutaneous ganglion of X, which 

 was described by Coghill, is the indication of a placode in the 

 ectoderm which gives rise to that ganglion. This fact falls in 



