STUDIES IN MAMMALIAN EMBRYOLOGY. 397 
diagram 28. Fig. 39 is between diagrams 28 and 29. Figs. 40—42 cor- 
respond to diagram 29. Fig. 43 corresponds to diagram 382 (area vasculosa). 
Figs. 44. and 47 correspond to diagrams 33 and 34. Figs. 45 and 46 cor- 
respond to diagrams 35 and 36. Fig. 48 corresponds to diagram 35. Figs. 
49 and 50 correspond to diagram 36. Figs. 51 and 52 correspond to diagram 
32. Fig. 53 is between diagrams 32 and 33. Fig. 54 corresponds to diagram 
33. Fig. 55 corresponds to diagram 34. Fig. 56 corresponds to diagram 35. 
Fig. 57 corresponds to diagram 36. 
In order not to overcrowd the diagrams I have purposely omitted the 
embryo itself in Figs. 33 —36, where the amnion enclosing it takes its place. 
It would have been all the more difficult to give an adequate representation of 
the embryo, as in the stages 32 and 33 there is a large pro-amnion, the head 
thus projecting considerably into the yolk-sac. Whereas in Fig. 32 the embryo 
is still symmetrically situated, it is twisted in Fig. 33, the tail and trunk being 
turned over about 90° at the time the allantois achieves its attachment to the 
trophosphere. In the stage of 34 the pro-amnion no longer projects; the 
whole embryo is now turned side upwards. In Fig. 35 it gradually turns 
again; and in Fig. 36 it has once more reached its original—now also its 
final—position, with its back towards the placental area. During these 
successive displacements the connecting string of tissue between the embryo 
and the yolk-sac remains very short; that between the allantois and 
the navel becomes long; and in the stages of Figs. 35 and 36 the allantoic 
vessels, united into a bundle, encircle one half of the embryo, taking their 
course just outside the amnion. ‘These vessels, as well as those that pass from 
the mesometrium towards the trophosphere or placentary region, have for the 
same reason been omitted in this figure. 
The diagrams here given may be briefly characterised as follows : 
Fic, 28.—Embryo still free in uterine lumen; folds of decidua reflexa as 
yet open; decidual swelling compact, not vascular, but vasifactive. 
Fig. 29.—Reflexa on the point of being closed; decidual swelling richly 
vascularised. 
Fic. 30.—Embryonic area formed ; trophosphere further advanced. 
Fic. 31.—Mesoblast formed; blastocyst definitely shut off from uterine 
lumen. 
Fic. 32.—Closure of amnion, formation of diplotrophoblast; area vasculosa 
spreading over the greater part of the yolk-sac; formation of omphaloidean 
diplotrophoblast ; trophosphere in full development, first origin of allantois. 
Fie. 33.—The allantois abuts against the trophoblast (and thin layer of 
somatic mesoblast), and becomes intimately connected with this, thus consti 
tuting the allantoidean diplotrophoblast. Yolk-sac, with considerable villi, 
enclosed between trophoblastic growths. 
Fie. 34.—Allantoidean diplotrophoblast further developed. Yolk-sac 
gradually losing attachment in trophosphere as allantois increases and amnion 
