346 F. L. LANDACRE 



At this time the future ganglion consists of two constituents, 

 first a placodal constituent proliferated from the thickened epi- 

 dermis or epibranchial placode which is largest in its middle 

 region and becomes smaller both posteriorly and anteriorly and is 

 everywhere still attached to the epidermis although it later be- 

 comes completely detached. Secondly there is a lateral mass 

 constituent lying on the dorso-mesial portion of the placodal 

 constituent and ending posteriorly in loose lateral mass cells so 

 that at this stage its posterior boundary cannot be definitely 

 determined. 



As the geniculate ganglion becomes larger the difference in size 

 between the body of the ganglion and its root enables one to deter- 

 mine approximately the posterior extremity of the ganglion. 

 This can be done with certainty only when the fibrillated root 

 appears. At this time the trigemino-facial complex has the ar- 

 rangement indicated in fig. 83 when the Gasserian and geniculate 

 ganglia overlap for about one-half the length of the Gasserian. 

 This overlapping is due apparently mainly to the forward growth 

 of the geniculate on the mesial side of the Gasserian, the root of 

 the Gasserian remaining constant in position and the body of the 

 ganglion extending only slightly posterior to its original position. 

 The forward extension of the geniculate is probably entirely due 

 to growth in size of the ganglion, since I can find no evidence of 

 the further proliferation of cells anterior to the point occupied 

 by the placode. 



Both the Gasserian and geniculate ganglia assume a definite 

 form with well defined boundaries rather slowly, so that for some 

 time they consist of condensed masses of cells with irregular out- 

 line. Since the extent of fusion of the acustico-facial complex is 

 very great in the adult in Ameiurus, I have given a series of figures 

 (31 to 39) to show the condition of the ganglia at a time when all 

 the components are still separate and the roots and chief trunks 

 of the nerves derived from these ganglia are still distinct. These 

 figures are taken from the same embryo (A. melas 86 hours) as 

 that from which fig. 83 was constructed. This plot should be 

 compared with fig. 2 of Herrick's paper ('01). The principal 

 point of interest here lies in the fact that the roots and chief 



