66 J. r. HILL. 



the same stasfe of development. They measured from 4-5 to 

 6 mm, in diameter. 



In tliis stao-e the formative region of the preceding blasto- 

 cysts has become transformed into the definitive embryonal 

 area (embryonic shield, Hubrecht) as the result of the com- 

 pletion of that process of inward migration of the entodermal 

 mother-cells which we saw in progress in the vesicles last 

 described, and the consequent establishment of the entoderm 

 as a continuous cell-layer underlying and independent of. the 

 embryonal ectoderm constituted by the larger passive cells of 

 the original unilaminar formative layer. 



In the entire blastocyst (PI. 4, fig. 39) the embryonal area 

 is quite obvious to the naked eye as the more opaque, hemi- 

 spherical region, forming rather less than half the entire 

 extent of the vesicle wall ; the larger remainder of the same 

 is formed by the much more traiisparent, non-formative or 

 extra-embryonal region. Sections of the entire blastocyst 

 show (1) that the embryonal area is bilaminar over its entii-e 

 extent, its outer layer consisting of embryonal ectoderm, 

 already somewhat thickened, its much thinner inner layer 

 consisting of entoderm, partly still in the form of a cellular 

 reticulum, and (2) that the extra-embryonal region is still 

 unilaminar throughout and composed of a relatively thin 

 layer of flattened cells (extra-embryonal or trophoblastic ecto- 

 derm, trophoblast [Hubrecht])! (l^i- 8, fig. 78). The entoderm 

 is co-extensive at this stage with the embryonal ectoderm, 

 and terminates in a wavy, irregularly thickened, free edge 

 (PI. 5, fior. 49), which over most of its extent eitlier directly 

 underlies or extends very slightly beyond the line of junction 

 between the embryonal and extra-embryonal ectoderm. The 

 junctional line is thus not very easily seen. In fig. 48, however, 



' In consonance witli my conviction that this layer is liomologous 

 both with the so-called trophol)lastof Eutheriaand the extra-emhryonal 

 ectoderm of Prototheria, and in view of the theoretical signification 

 which Huhrecht now insists should be attached to the term " tropho- 

 blast." and which I am wholly unal>le to accept, I venture to suggest as 

 an alternative name for this layer that of " tvopho-ectoderm. ' 



