488 L. S. STONE 



of shorter duration and, therefore, difficult to follow. The 

 earliest stage in which Coghill ('16) described the early contact 

 of the ophthalmic ganglion with the ectoderm corresponds to 

 about stage 34, i.e., a stage between those shown in figures 6 

 and 7. At the point of contact lies the placode which can be last 

 seen at this stage. The actual contribution of placodal cells to 

 the ganglion must be observed in earlier stages than this, at a 

 time when crest cells are still very numerous in this region. This 

 condition has been the factor which has caused investigators to 

 overlook the placodal contribution. Coghill's observation that 

 during this contact with the ectoderm the ganglion makes its 

 connection with the brain adds further morphological evidence 

 that the ganglion is of placodal origin. Judging from the many 

 similarities in Amblystoma and Necturus, it is obvious that Piatt 

 was correct in assuming an ectodermal contribution to the oph- 

 thalmic ganglion, although she has confused the placodal and 

 crest cells in this region and incorrectly interpreted part of these 

 placodal cells as contributing to the 'mesectoderm.' The ecto- 

 dermal cells which Goette ('14) describes being given off above 

 the optic vesicles in Siredon (Amblystoma tigrinum) and Torpedo 

 are likewise cells of the ophthalmic placode and not contributions 

 to the wandering 'ectomesoderm.' It has been shown that when 

 the ophthalmic placode is entirely removed in early stages of 

 Amblystoma there is a complete absence of the ophthalmic 

 ganglion and the ophthalmicus profundus V nerve (figs. 40 b and 

 43). Such cases always show but little disturbance of the crest 

 cells in the trigeminal region. In control operations in which the 

 ectoderm was removed in the usual manner and then replaced 

 there appeared normal ophthalmic ganglia and nerves, which 

 shows that the placode is necessary for the formation of the 

 ganglion. When a small portion of the ectoderm over the eye is 

 left a very small displaced ganglion is often found (fig. 45 b). In 

 this respect it is similar to other partially removed placodes. 

 On the other hand, when the ophthalmic placode is left intact 

 and the crest cells have been disturbed as much as possible by an 

 attempt to remove them, an apparently normal ganglion is pres- 

 ent, and although there has been a regeneration of the crest cells 



