122 EDWARD L. RICE 
posterior margin of the tympanum; the body is measured from 
the posterior margin of the tympanum to the anus. The meas- 
urements of the tail and of the total length are much less reliable 
than the other data, owing to the fact that the tip of the tail of 
almost every embryo was damaged in the preparation of the 
material. 
Tabular statement of size and age of embryos studied 
LENGTH IN MILLIMETERS 
STAGE AGE IN DAYS 
Head Body Tail Total 
1 Se 3.25 6.50 7.00 16.75 
2 x+2 3.50 7.00 7.50 18.00 
3 x+4 4.00 8.00 " 2 
4 x+6 4.50 9.00 is ? 
5 x+9 die acts) 9.75 13.50 28 .00 
6 x +17 6.00 13.00 20.00 39 .00 
The modification in form during the seventeen days of devel- 
opment is shown in figures 29 to 31, which are drawn to scale by 
means of the camera. In stage 1 the chondrocranium is already 
well mapped out, although much of the cartilage is in a very 
young condition and should, perhaps, be designated as procar- 
tilage or even blastema. Bone formation is merely suggested— 
pterygoid, extreme anterior end of dental, and very thin dentine 
layers in the teeth. Stage 2 shows a marked advance in bone 
formation, and in stage 5 the full complement of membrane bones 
may be recognized, with the exception of the lacrimal and para- 
sphenoid. The lacrimal appears in stage 6; the parasphenoid 
is still wanting in this the latest of my embryos. There is no 
actual replacement of cartilage by cartilage bone until stage 6 
is reached, although the bone formation is anticipated in stage 5 
by a marked histological modification of the cartilage in many 
places. Even in stage 6 the replacement of cartilage by bone 
is only begun, the most conspicuous ossification centers being 
found in the posterior part of the basal plate, the occipital arches, 
the posterior parts of the quadrate and Meckel’s cartilage, and 
some portions of the otic capsule and hyoid. In stage 6 the 
