ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
437 
blastocyst and the yet unaltered decidual stroma is named the troplio- 
spongia, while the embryonic trophoblast plus the maternal tropho- 
spongia, forming in Erinaceus a sphere shut off from the lumen of the 
uterus, is called the trophosphere. In this there appear certain specially 
large cells termed deciduofracts, the name suggesting their plausible 
function. While the author believes that trophospongia and deciduofracts 
arise from the endothelium of the blood-spaces of the decidual swelling, 
close to the blastocyst, he does not exclude the possibility that cells 
belonging to the decidual stroma may have a subordinate part in forming 
the trophospongia. The rest of this chapter is devoted to a considera- 
tion of the allantoidean trophospongia, the outer decidual layers, and the 
details of the mucosa. 
III. Physiology of Placentation . — At a very early stage, the lacunas 
of the blastocyst wall are filled with maternal blood, which contributes 
to growth and development. This primitive arrangement is succeeded 
by a very effective omphaloidean placentation, in which there is only a 
thin partition between the vitelline circulation and the maternal blood 
filling the trophoblastic spaces. This declines, however, as the final 
allantoidean placentation is established. The yolk-sac ceases to grow, 
and is folded up, although its circulation never wholly disappears ; the 
trophoblast, against which it was applied, becomes membranous along 
with the rest of the omphaloidean trophosphere and the decidua reflexa. 
Of the vascular outgrowths of the allantois, as of those of the yolk-sac, 
it is true that they on no occasion penetrate or grow into maternal tissue. 
It is embryonic (trophoblastic) tissue that carries the maternal blood to 
them. The deciduofracts j>ossibly act like phagocytes, with a direct 
destructive influence on the mucosa. 
Prof. Hubrecht then reviews some recent contributions to the history 
of placentation, and urges against Sir William Turner, “ that grand-master 
of placental research,” four conclusions : — (1) In numerous orders 
(Carnivora, Chiroptera, Rodentia, Insectivora), the maternal epithelium 
disappears at a very early moment where the blastocyst adheres ; (2) in 
the more primitive of the above orders, lacunar blood-spaces are in 
direct contact with the blastocyst long before the embryonic area 
vasculosa appears ; (3) the connection between these lacunae and the 
maternal blood-vessels is brought about in a more indirect way than by 
mere dilatation of capillary vessels ; (4) in later stages, foetal epiblast in 
varying thickness is present between the omphaloidean or allantoic 
villi and the maternal blood ; in Insectivora, Chiroptera, Rodentia, this 
trophoblast is the only tissue so intervening. The author proposes to 
abandon the distinction between deciduate and indeciduate placentation, 
and maintains that the Insectivora furnish the natural starting-point for 
the placental series. After some observations on the ventral stalk 
(Bauchstiel of His), and other features in human placentation, Hubrecht 
concludes his elaborate memoir with a tabular comparison of the various 
names given by different investigators to placental structures. 
Acquisition and Loss of Food-yolk, and Origin of the Calcareous 
Egg-shell.* — Mr. J. A. Ryder outlines his theory of yolk and shell. In 
primitive types which have ova almost wholly without yolk, surplus 
* Amer. Natural., xxiii. (1889) pp. 928-33. 
