160 FRANCIS VILLY. 



The investigations here recorded have been conducted almost 

 entirely by means of transverse sections, longitudinal sections being 

 examined when points of special difficulty were encountered. 



Before passing on to the immediate subject of this paper I wish 

 to take this opportunity of thanking Professor Marshall for the help 

 that he has so readily given me, without which my work would 

 have been lengthened, and rendered much more laborious than it 

 has proved to be. 



Early Stages. — The earliest stage at which the auditory involution 

 can be readily recognised occurs soon after the closure of the neural 

 groove in the fore part of the embryo. It will be seen from Fig. 2 

 that the invagination, though shallow, is perfectly distinct, the epi- 

 thelium lining it being somewhat sharply marked off from the 

 surrounding epiblast by the columnar character of its constituent 

 cells. The invagination only concerns the deeper or nervous layer 

 of epiblast, and is placed in the longitudinal groove separating the 

 lower part of the embryo, distended with yolk, from the neural 

 surface, and a little above the level of the notochord. The edge of 

 the pit is roughly circular, and as yet the involution is regular, 

 being deepest in the centre, and shallowing in all directions towards 

 the edge. 



Although I have taken this stage as the earliest in the develop- 

 ment of the ear, the auditory epithelium may be recognised with 

 more or less certainty before the neural groove closes in. In these 

 very early stages the deeper layer of epiblast is somewhat thickened 

 in the region where the ear is formed later, and the auditory nerve 

 may already be traced into continuity with it. 



As development proceeds the involution is more rapid at the 

 dorsal than at the ventral part, so that in section it comes to have 

 the appearance shown in Fig. 2. After this has taken place the 

 ventral part in turn grows rapidly, pushing its way inwards (see 

 Fig. 3), and as this process is going on the lips of the pit grow 

 towards each other and coalesce, thus closing the aperture completely. 

 Soon afterwards the newly formed outer wall of the vesicle separates 

 from the deeper layer of epiblast, which remains near the surface. 

 I am not certain that the mouth of the auditory sac is closed 

 precisely in this manner, for the nervous layer of epiblast is so ill 

 defined at this time and place that I have thought it more probable 



