CRANIAL GANGLIA OF AMBLYSTOMA 459 



tion in the region of the hind-brain lends itself as a landmark in 

 orienting similar operations upon embryos from the closure of 

 the neural folds to stage 25 (figs. 1 and 2). 



A. Extirpation of placodes 



1. Removal of ophthalmic placode. A triangular piece of ecto- 

 derm, including skin dorsal to the optic vesicle, was removed in 

 the manner shown in figure 38 and indifferent ectoderm was 

 grafted upon the denuded area. Such areas of ectoderm were 

 taken from embryos varying in stages from 23 to 26. 



A typical picture of the extension of the implanted blue ecto- 

 denn may be obtained from figure 39. Gradually the blue area 

 extends ventrally until it includes the ectoderm over the eye and 

 the regions anterior and posterior to it, always showing the more 

 extensive migration towards the ventral half of the mandibular 

 arch and towards the first external gill. As a result of this, the 

 blue ectoderm several days after the operation comprises an 

 area far greater in extent than the original extirpated area. 

 After seven or eight days the Nile-blue sulphate gradually dis- 

 appears and the transplanted area can often be located by its 

 scarcity of pigment — a characteristic of the ventral ectoderm of 

 older embryos. The same condition of the extension of blue 

 implanted ectoderm may be seen in many other cases (figs. 48, 

 49, 55, 63) . This may be largelj^ due to a migration of the super- 

 Fig. 38 Showing triangular piece of ectoderm, indicated by dotted line and 

 middorsal line which was excised in order to remove the ophthalmic placode at 

 stage 23. X 10. 



Fig. 39 Camera-lucida drawing of the same individual a few days after opera- 

 tion, showing extent of migration of implanted ectoderm stained in Nile-blue 

 sulphate. X 10. 



Fig. 40 a and b Showing, respectively, lateral views of a reconstruction in the 

 trigeminal region of the normal and operated sides. Only a small gasserian 

 ganglion appears on the operated side. X 50. 



Figs. 41 and 42 Frontal sections of the specimen reconstructed in figures 

 40 a and b comparing the normal and operated sides at levels of the root of V. 

 X 37. 



Fig. 43 Frontal section of specimen killed nineteen days after operation, 

 showing on right side only the gasserian portion of V after the ophthalmic placode 

 has been excised. X 37. 



