342 F. L. LANDACRE 



THE ORIGIN OF THE GENICULATE GANGLION 



Owing to the fact that both in point of time and in position 

 there is such a close relation between the epibranchial ganglion 

 of the Vllth nerve and the disappearing preauditory placode and 

 that the cells derived from the epibranchial placode combine with 

 cells from the lateral mass to form the definitive communis or 

 geniculate ganglion of the Vllth nerve, it will be more conven- 

 ient to describe the origin of the geniculate ganglion here than to 

 follow the natural order and take up the differentiation of the post- 

 auditory lateral mass. The series of figures (19 to 24, Stage VI) 

 is taken from an embryo slightly older than the preceding, and 

 in it the last trace of the preauditory placode has disappeared 

 and the thickening which later gives rise to the epibranchial 

 Vllth is just forming. Fig. 19 is taken at the point of contact 

 of the hyoid endodermal pocket, with the epidermis and corre- 

 sponds in position to fig. 18. It differs in tw T o important respects; 

 the epidermis dorsal and ventral to the contact with the endoderm 

 shows no resemblance to the preauditory placode but is composed 

 of cells whose outlines are very indistinct and whose nuclei are 

 irregularly arranged. In addition to the disappearance of the 

 placode in the region just mentioned, at the point of contact with 

 the endoderm, the epidermis is closely fused with it and presents 

 the appearance of being about to disappear entirely. This gill 

 slit does not open completely in any of my series but the two layers 

 of endoderm do separate in one of them, presenting the exact 

 appearance of an open gill pocket. Fig. 20 is taken four sections 

 posterior to fig. 19 at the point where the contact of the endoderm 

 with the epidermis ceases and it is characterized by the irregular 

 arrangement of its epidermal nuclei, some of which appear to have 

 been proliferated mesially but are still in contact by cytoplasmic 

 strands with the epidermis. Figs. 20, 21, 22, 23 are consecutive 

 sections and lie just behind the point of contact of the gill pocket 

 with the epidermis. In figs. 21 and 22 a proliferated mass of 

 cells, quite small, is isolated slightly from the epidermis. The 

 earliest stage of the epibranchial placode of the Vllth nerve is 

 indicated by the thickened epidermis shown in these figures. 



