990 
used to differentiate the bone and cartilage. In cases where decal- 
cification was necessary, celloidin was employed for the embedding 
mass, and the same combination of stains was used. 
Descriptions. 
First Stage of Development. (Figs. 1—14.) 
Embryos of 5 and 6 mm. total body length, although too young 
to show any recognizable skeletal condensations in the ear region, 
are interesting because of the form of the first visceral cleft. The 
median region opens broadly into the pharynx. The lateral area is 
composed of two parts; one, 
the larger dorsal region, is of 
the expected form, a latero- 
caudal slit opening to the 
surface, and the other, in the 
ventral region, an abrupt 
latero-anterior outpushing 
which ends blindly a short 
distance below the epithe- 
lium. In later stages when 
the parts of the skeleton 
become differentiated the 
reason for such a form is ap- 
parent. 
An embryo of 7 mm. 
Fig. 1. Section of an embryo of 7 mm. total ion ds 
body length showing the general morphological en 2) 
relations of the first stage of development. presents the earliest stage in 
ch.ty. chorda tympani; f. arteria facialis. which the precartilage con- 
ggl.fac. ganglion faciale; h.s. head somite; : 
l.v.p. first visceral pouch; m.lab. membranous densations of the skeletal 
labyrinth; p.col. proton of columella; Vil re. elements can be recognized. 
ramus dorsalis nervus facialis. . R 
Although their boundaries 
are by no means definite, 
still they can be easily identified from their relations to one another 
and to the visceral clefts, nerves, and blood vessels which are more 
clearly defined. 
Taking, as a land-mark, the first visceral pouch which is still 
open, we find anterior to its ventral outpushing in the mandibular 
arch, a densely packed mass of cells in which we recognize the proton 
of the quadrate and mandible. Behind the same pouch, in the second 

