STUDIES IN MAMMALIAN EMBRYOLOGY. 401 
and has become a closed spherical investment all round the blastocyst. (Mus. 
Utr. Cat. n® Erin. 95 d, 2 7, 4s.) 
PLATE XXIV. 
Fic. 43.—First stages of the extension of vitelline circulation on the yolk- 
sac against the lacunar trophoblast and trophospongia. Zr. The cellular 
trophoblast with the lacunary spaces (sp.) between the trophoblastic villi. 
Tr. s. The trophospongia of which certain of the outer cells become converted 
into larger deciduofracts (df). m.som. A thin layer of somatic mesoblast, 
situated between the trophoblast and the splanchnic mesoblast of the yolk-sac. 
m. som. is continued into the somatic mesoblast of the outer amnion fold (ef. 
Fig. 51). m.sp. The splanchnic mesoblast of the yolk-sac with the vitelline 
vessels, carrying embryonic blood-corpuscles. Ay. The inner hypoblastic layer 
of the yolk-sac. (Utr. Mus. Cat. n® Erin. 284 a4, 2 7.) 
Fie, 44.—The same region in a later stage. The trophoblast has proliferated 
into an arachnoideal arrangement of blood-spaces; the yolk-sac has become 
villiferous, and part of these villi have again been enucleated. ‘To the left of 
the figure is seen the yolk-sac, with blood-vessels (v.) and with vascular super- 
ficial villi (vz.). One of these, fitting in between the trophoblastic tissue (Z’.), 
with its spacious blood-lacunee (sp.) (filled with maternal blood-corpuscles), is 
just extracted out of its nutritive surroundings ; another, lower down, is still 
in its place. Between trophoblast and trophospongia (Zr. s.) a thin layer of 
more fusiform cells (s/.) has become visible. To the right of the trophospongia, 
with its blood-lacunz, we notice the layer of deciduofracts (df). Still further 
to the right is the decidual tissue (D.), of which the inner layer (D’.) has 
assumed a more reticular aspect. Maternal blood passes from the vessels of 
the compact decidua (not one of them happens to be cut in this section), 
through all the layers noticed, into the spaces of the trophoblast. The somatic 
mesoblast is not recognisable as a distinct membrane in this preparation. (Mus. 
Utr. Cat. n Erin. 54 2°, 5s.) 
Fic. 45.—The reciprocal relation of yolk-sac and trophosphere in much later 
stages of pregnancy. In the enucleated yolk-sac the vitelline vessels are marked 
v., the hypoblastic lining Ay. Trophoblast and trophospongia are thinned 
out into a membrane (77. and Zr. s.), which is inwardly coated by a hyaline 
layer (4. Z.) that has developed (cf. fig. 48) between the layers just mentioned and 
the layer of somatic mesoblast (m. som.), the cellular elements of which have 
again assumed a more voluminous aspect after the enucleation of the yolk-sac 
out of the diplotrophoblast. (Mus. Utr. Cat. n® Erin. 209 a’, 27. 8 s.) 
Fic. 46.—A section of the tissue of the yolk-sac in still later stages of 
pregnancy, when it is folded against the belly of the animal (cf. diagram 36), 
and has undergone considerable thickening of its walls. v. Blood-vessels in 
