No. 8. Rudiment of the third groove (_gr_3.). No. 9. Müllerian duct quite free in the space between the peritoneal epithelium and the Wolffian duct, in which condition it extends until near its termination. Between Nos. 9 and 10 is an interval of eight sections. No. 10. The penultimate section, in which the Müllerian duct is seen. A lumen cannot be clearly made out. No. 11. The last section in which any trace of the Müllerian duct is visible. No line of demarcation can be seen separating the solid end of the Müllerian duct from the ventral wall of the Wolffian duct. FIGS. E. and F. Sections through the glomerulus of the head-kidney from an embryo prior to the appearance of the head-kidney. Zeiss B, ocul. 2. A comparison of the two figures shows the variation in the thickness of the stalk of the glomerulus. E. Section anterior to the foremost Malpighian body. F. Section through both the glomerulus of the head-kidney and that of a Malpighian body. The two are seen to be connected. SERIES H. Consecutive sections through the hind end of the Müllerian duct, from an embryo in which the head-kidney was only represented by a rudiment. (The embryo was, perhaps, very slightly older than that from which Series D was taken.) Zeiss C, ocul. 3 (reduced one-third). No. 1. Müllerian duct is without a lumen, and quite distinct from the Wolffian wall. No. 2. The solid end of the Müllerian duct is no longer distinct from the internal wall of the Wolffian duct. No. 3. All trace of the Müllerian duct has vanished. SERIES I. Sections through the hinder end of the Müllerian duct from an embryo of about the middle of the sixth day. Zeiss C, ocul. 2 (reduced one-third). No. 1. The Müllerian duct is distinct and small. No. 2. Is posterior by twelve sections to No. 1. The Müllerian duct is dilated, and its cells are vacuolated. 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. EXPLANATION OF PLATE 29. (XIV. p. 644.) COMPLETE LIST OF REFERENCE LETTERS. _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.]