STUDIES IN MAMMALIAN EMBRYOLOGY. 403 
with it the diplotrophoblast. In the splanchnic mesoblast the vitelline 
circulation is already well developed. (Mus. Utr. Cat. no Hrin, 156’ 3, 
27, 9s.) 
Fig. 52.—A similar section as the foregoing, after the completion of the 
amnion. (This figure was taken from the same specimen of which the 
vitelline circulation is represented in Fig. 43.) 2. am. and mes. am. 
Epiblastic and mesoblastic layer of the amnion. m., som. and Zr. As 
in Fig. 51. sp. Blood-spaces between the trophoblastic villi. Z7r.s. 
Trophospongia. The trophoblast does not as yet show traces of active 
proliferation. (Mus. Utr. Cat. n° Hrin. 284 a3.) 
Fic. 53.—The same region in a stage somewhat further advanced, in 
which, however, the protruding allantois has not yet traversed the 
ccelomic space, and has not yet come in contact with the wall of the 
blastocyst. Active proliferation has commenced in the trophoblastic 
tissue (77.), which is similarly altered (as was noticed in Fig. 44 in the 
vitelline region) into an arachnoidal network enclosing spacious lacunx 
(sp.) derived from the spaces (sp.) of Fig. 52,and filled with maternal 
blood through the intermediary of the trophospongia (Z7s.). Between 
the latter tissue and that of the trophoblast a layer of fusiform cells (s/.) 
is interposed. Further downwards this layer is directly continued into 
the layer (s/.) of the vitelline region, which is represented in Fig. 44. 
m. som. A thin layer of somatic mesoblast. (Mus. Utr. Cat. n® Erin. 
155e, 27. 8s.) 
Fig. 54.—The same region at a later stage, when the allantois has just 
come into contact with the diplotrophoblast. The latter has still further 
proliferated (the scale of enlargement is the same as in the preceding 
figures), and in the recesses between its more prominent portions the 
allantois-villi can easily grow and develop without any active pene- 
tration on their part. Two of these villi, marked a. v., are seen in this 
figure. The blood-vessels in the allantois (a//.) are very distinct, and 
filled with embryonic blood-corpuscles. The hypoblastic lining of the 
allantois (Ay. a.) is also still a distinct feature in this stage. Zr. The 
trophoblast with the blood-spaces (sp.); the latter distinctly com- 
municating (e.g. in the middle of the figure) with those in the tro- 
phospougia (Z7s.), through interruptions in the more stratified layer 
(st.). In this preparation only faint traces of the somatic mesoblast as 
a distinct constituent of the diplotrophoblast can be detected. (Mus. 
Utr. Cat. n° Erin. 45 a, 5s.) 
