142 Development of Ear and VII-VIII Cranial Nerves 
capsule, and become incorporated in the underlying head skeleton, the 
communication with the surface being maintained by a specially devised 
accessory apparatus. 
In the embryo the first sign of the auditory organ, according to 
Krause, 03, and Poli, 97, consists of a thickening of the ectoderm, the 
auditory plate, which is seen lateral to the still open medullary groove 
in the region of the future third brain vesicle. In vertebrates having two 
layers of ectoderm the thickening involves the inner layer, the outer not 
being affected. Owing to the fact that the growth of cells shows greater 
activity in the deeper strata of the auditory plate it soon becomes con- 
verted into a cup shape depression and is then called the auditory fossa 
or auditory cup. By the folding in and closure of its edges the auditory 
cup is in turn converted into the auditory vesicle, which, however, 
remains attached to the surface for a longer or shorter period by means 
of an epithelial stalk or canal being finally separated from the surface, 
in mammals much earlier than in lower vertebrates. 
It is at this point, just after the ear vesicle has been pinched off from 
the ectoderm, that my own observations begin. This stage corresponds 
to the “ primitive ear vesicle” of Krause, 03, and will be described under 
that heading here. 
The primitive ear vesicle-—The reconstruction of the ear vesicle of an 
embryo 4.3 mm. long, No. 148, shown in Fig. a, Plate I, represents 
our youngest stage. This is considerably younger than the youngest 
human embryo described by His, Jr., 89. It is about the same age 
as shown in Krause’s, 03, Fig. 82, a model from a rabbit embryo, and is 
younger than the first stage of the series of models of the ear vesicle of the 
bat recently published by Denis, 02. 
The ear vesicle consists at this time of a slightly elongated, oval sac, 
having the following diameters: dorso-ventral, .39 mm.; caudo-cephalie, 
:26 mm., and transverse, .28 mm. It lies closely against the neural tube, 
and is connected with it by the acoustic ganglion, similarly as is shown by 
Mall, 88, in the dog, figured in his Fig. 4, Plate XX, and is surrounded 
on all sides by a thin layer of mesodermal tissue. 
On the dorso-lateral surface, above that portion which is to become 
vestibular pouch and near where the endolymphatic appendage is to be 
separated off from the rest of the vesicle, there is a shallow groove. This 
groove, as seen in the sections, is cut transversely and consists of a seam, 
or the meeting point of the former edges of the auditory cup whose 
approximation completes the closure of the vesicle. This closure seam 
shows various degrees as regards the completeness of fusion, manifested 
