v 
( 
Normal plates of the development of the rabbit (Lepus cuniculus L.). 8 
compared with the latter; the entoderm also is slightly thicker. This anterior portion of the shield is two- 
layered, ectoderm and entoderm. H.E.C., No. 625. 
0 Figs. 3 (X 20) and 15 (X 5). 
The drawing is a reproduction from another drawing made from a specimen which has since been 
cut. Embryo removed from uterus 7!/, days after coitus. ZENKER fixation. Mesoderm measured 4.0 X 3.4 mm. 
The embryonic shield is pear-shaped. HENSEnN’s knot, situated a little anterior to center of shield, is distinct. 
Notochordal anlage is visible as a more opaque band extending forward from HENnsEn’s knot. Primitive 
groove runs from HENsEn’s knot to posterior end of shield. A more opaque area, corresponding to the 
extent of the mesoderm, stretches for some distance around the shield. This area reaches the anterior 
margin of the shield but does not pass in front of it. Over this area, beyond the boundaries of the embryonic 
shield, the outer ectodermal layer (trophoblast of HUBRECHT) is somewhat thickened. There is a distinct 
projection at Hensen’s knot. The embryonic ectoderm over the region of the notochordal anlage is a little 
thinner than laterally in this part of the shield. The primitive groove is very shallow. H. E. C., No. 622. 
Be Figs. 4 (X 20) and 16 (X 5). 
we Specimen removed from uterus 8 days, 6 hours after coitus. TELLYESNICKY fixation. Measured 
1.8 mm. from anterior end of embryonic shield to posterior end of primitive streak. No segments. Embryonic 
shield broader at anterior end than at posterior. Hensen’s knot is distinct: it is situated a little posterior 
to center of shield. The primitive groove extends from Hansen’s knot to the posterior end of the shield, 
ending there in an opaque spot. The notochordal anlage is faintly indicated as a more opaque band 
extending forward from HEnsen’s knot. The ectoderm of the lateral portion of the shield, anterior to the 
region of Hensen’s knot, is somewhat thicker than the median ectoderm over the notochordal anlage. This 
apparently marks the beginning of the medullary plate with a suggestion of a groove in the center. There 
is no projection at Hensen’s knot. There are apparently no blood anlagen. This specimen is only a little 
more advanced than that of Fig. 3. 
„3 8 Figs.5 (X 20) and 17 (X 5). 
er Embryo removed from uterus 8 days, 6 hours after coitus. ZENKER fixation. Measured 2.2 mm. from 
anterior end of medullary plate to posterior end of primitive streak. (The specimen was, however, a little 
bent, concave dorsally.) Medullary plate distinct, elongated, flat, slightly expanded at anterior end on each 
side in a curved manner. Posteriorly it surrounds HEnseEn’s knot and merges into region of primitive 
streak. Medullary groove is relatively wide and shallow. The notochordal anlage is visible through the 
groove. The primitive streak is considerably shorter than the medullary plate. The primitive groove is 
very shallow. HENnsEn’s knot conspicuous as a circular opaque spot. Only one pair of segments is clearly 
marked off on both sides, but a second pair, posterior to the former, is clearly indicated though not sharply 
bounded on the caudal side. The segments lie under the narrowest part of the medullary plate, nearer 
Hensen’s knot than the anterior end of the plate. There are a few small extra-embryonic blood anlagen. 
There is apparently no coelom. 
ED 8 Figs. 6, (X 20). and 18 (X5). 
„ge Embryo removed from uterus 8 days, I hour after coitus. ZENKER fixation. Measured 2.6 mm. from 
anterior end of medullary plate to posterior end of primitive streak. (Specimen was, however, a little bent.) 
Medullary plate distinct, quite flat, expanded cephalad in two lateral projections; narrowest in region of 
segments; surrounds HENSEN’s knot and merges into region of primitive streak. Medullary groove is wide. 
jr 
