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EICHAED ASSHETON. 
EXPLANATION OF PLATE 33, 
Illustrating Mr. Eichard Asslieton’s paper on “ Tropidonotus 
and the ‘ Arclienteric Knot ’ of Ornithorhynchus.” 
Fig. 1. — Outline figure of Tropidonotus embryo, showing three gill- 
cdefts, allantois, etc. all. Allantois, lit. Heart. 
Fig. 2. — Transverse section theough the central region of the lower 
pole of the egg of Tropidonotus after complete envelopment of the yolk 
liy the blastoderm of the stage of fig. 1. The loose yolk has been 
washed away, leaving hypoblast and germinal wall as a thickening 
round the fused edge of the epiblast. c. p. Heaped-up rim of coalesced 
ei^iblast. e.p. Epiblast. l\y. Hypoblast with much yolk. (j. w. Genninal 
wall. y. Pure yolk not yet covered by, or converted into gemiinal 
wall. 
Fig. 3.^ — Diagram of the above, lettered and drawm after the manner 
of Wilson and Hill's text-fig. 4, with which it should be compared, 
c.p. Cell-plug of ectoderm. ecA Ectoderm. e«t. Endoderm. c.z. (Central 
zone) part of central phig of ectoderm obviously in continuity with the 
ectoderm, m. z. (Marginal or cortical zone) -- germinal wall, or hypo- 
blasts and yolk. y. Yolk. 
Fig. 4. — Another section of the same series of transverse sections 
through the central region of the lower pole of Tropidonotus. The 
section passes through a little cleft still remaining between the edges 
of the coalesced blastodenn rim. c. p. Plug of cells derived from the 
coalesced rim of the blastoderm, e. p. Epiblast. hy. Hypoblast, g. w. 
Germinal wall. y. Yolk. 
Fig. 5. — A portion of the section shown in fig. 4, to illustrate the 
cellular character of the “ cell-plug ” (c. p.) (coalesced blastoderm edge), 
and e.p. epiblast, hy. germinal wall (hypoblast). 
