4 Normentafeln zur Entwicklungsgeschichte der Wirbelthiere, 
Notochordal anlage is visible through medullary groove. HENnsen’s knot is distinct as a circular opaque 
spot. The primitive streak is short. There are three distinct pairs of segments, but the third pair is not 
completely separated on the posterior aspect. A fourth pair, anterior to these three, is indicated, but is 
small and not clearly marked off. There are a few extra-embryonic blood anlagen and primitive blood cells. 
There is a very small coelom. 
Figs. 7 (X 20) and 19 (X 5). 
The drawing is a reproduction of another drawing made from the specimen before sectioning. The 
embryo was removed from the uterus 81/, days after coitus. ZENKER fixation. Measured 3.4 mm. The 
medullary groove is wide open. There are six distinctly formed pairs of segments; the seventh pair caudad 
is nearly separated. Primitive streak distinct. The caudal end of the embryo in the region of the primitive 
streak is somewhat bent. For internal development, see Table No. 2, made from a study of the sections 
of this embryo. 
Ü Figs. 8 (X 20) and 20 (X 5). 
Embryo removed from uterus 81/, days after coitus. ZENKER fixation. Measured 3.2 mm. from tip 
of head to caudal amnion. There are eight distinctly formed segments. Posterior to the eighth, a ninth 
is almost completed. The cephalic end of the embryo is raised above the level of the surrounding extra- 
embryonic disk but dips ventral a little into the proamnion. The back is flat. The medullary groove is 
open throughout. It is considerably expanded in the region of the optic diverticula; a little expanded in the 
region of the hind-brain; nearly closed in the region of the future mid-brain. The walls of the medullary 
groove, between the segments, approach each other, but, posterior to the segments, diverge to form a space 
in which the remnant of the primitive streak is seen. Laterad of the segments, as seen by transmitted 
light, is a narrow longitudinal light band, where there is a very small amount of mesoderm. Again laterad, 
is a broader, darker area, where the mesoderm is thicker and incloses the coelom. Posteriorly, the segments 
pass into an unsegmented band. The proamniotic area is distinct. The caudal fold of the amnion has 
begun. The area of extra-embryonic ectoderm, which was atteched to the uterus and torn off on the 
removal of the specimen, is quite large. It reaches anteriorly as far as the plane of the hind-brain. On 
ventral view, the mesodermal allantoic fold is plain. The pocket of the fore-gut is just discernible. The 
notochord is not to be made out on external examination. Compare, for internal development, Tables 
Nos. 3 and 4. 
g 710891X 20), 2X Sand 21a O5) 
Embryo removed from uterus 9 days after coitus. ZENKER fixation. Measured 3.4 mm. There are 
ten distinct segments. The cephalic end of the embryo bends ventrad beneath the level of the extra- 
embryonic disk at an angle of nearly 135 with the rest of the body, and is buried in the proamnion. Posterior 
to the point of this bend, the body is flat. The place of entrance into the embryo of the vitelline veins is 
distinctly marked. As in the 81/,-day embryo, there is to be seen by transmitted light a narrow longi- 
tudinal light band, laterad of the segments. Again laterad, a broader, darker area. Posteriorly the segments 
pass into an unsegmented band. The proamnion, which encloses the head, reaches approximately to the 
region of the hind-brain. The caudal amniotic fold extends cephalad more than one third the length of the 
embryo. The medullary tube appears closed throughout except perhaps in the extreme posterior end. The 
walls of the hind-brain are a little expanded. The shallow otic pits are just visible alongside of the hind- 
brain. On ventral view, the heart chamber makes a slight projection under the head. The heart tube is 
