No. 3. Penultimate section, in which the Müllerian duct is visible; it is separated by three sections from No. 2.
No. 4. Last section in which any trace of the Müllerian duct is visible; the lumen, which was visible in the previous section, is now absent.
No. 5. No trace of Müllerian duct. Nos. 3, 4, and 5 are consecutive sections.
FIG. K. Section through the hind end of the abdominal opening of the Müllerian duct of a chick of 123 hours. Zeiss C, ocul. 2 (reduced one-third). It illustrates the peculiar cord connecting the Müllerian and Wolffian ducts.
_am._ Amnion. _ch._ Notochord. _ch´._ Notochordal thickening of hypoblast. _ep._ Epiblast. _hy._ Hypoblast. _m.g._ Medullary groove. _me.p._ Mesoblastic plate. _ne._ Neurenteric canal (blastopore). _pr._ Primitive streak.
SERIES A. Sections through an embryo shortly after the formation of the medullary groove. x 120[4].
Footnote 4: The spaces between the layers in these sections are due to the action of the hardening reagent.]
Fig. 1. Section through the trunk of the embryo.
Figs. 2-5. Sections through the neurenteric canal.
Fig. B. Surface view of a somewhat older embryo than that from which Series A is taken. x 30.
SERIES B. Sections through the embryo represented in Fig. B. x 120.
Fig. 1. Section through the trunk of the embryo.
Figs. 2, 3. Sections through the hind end of the medullary groove.
Fig. 4. Section through the neurenteric canal.
Fig. 5. Section through the primitive streak.
Fig. C. Surface view of a somewhat older embryo than that represented in Fig. B. x 30.
EXPLANATION OF PLATES 30, 31, 32. (XVII. p. 668.)
_ch._ Cheliceræ. _ch.g._ Ganglion of cheliceræ. _c.l._ Caudal lobe. _p.c._ Primitive cumulus. _pd._ Pedipalpi. _pr.l._ Præoral lobe. _pp_{ 1}. _pp_{ 2}. _etc._ Provisional appendages. _sp._ Spinnerets. _st._ Stomodæum.
I-IV. Ambulatory appendages. 1-16. Postoral segments.
Fig. 1. Ovum, with primitive cumulus and streak proceeding from it.
Fig. 2. Somewhat later stage, in which the primitive cumulus is still visible. Near the opposite end of the blastoderm is a white area, which is probably the rudiment of the procephalic lobe.
Fig. 3_a_ and 3_b_. View of an embryo from the ventral surface and from the side when six segments have become established.