EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 9 
the embryonic knob, the remaining cells of the latter form 
> which is thus situated between 
the entoderm and the trophoblast, and could for that reason 
easily deceive van Beneden (’80) in regarding it as a third, 
mesodermic layer (Figs. 8, 11, 12, 15, 17, 18, 21—23). 
the “embryonic ectoderm,’ 
3. Developmental phases of the Didermic Embry- 
onic Shield. 
The portion of the mammalian blastocyst where the em- 
bryonic ectoderm and its subjacent entodermal layer are situ- 
ated may, already at this early stage, be conveniently termed 
the embryonic shield. This shield is sometimes slightly 
convex with the ectoderm on the convex side (rabbit, Fig. 23), 
sometimes it is bent the other way (Sus, Fig. 17; Tupaja, 
Figs. 29—31; Tarsius, Figs. 20, 45), sometimes first one way 
(Figs. 20, 37, 53), and later (Figs. 50, 38) the other (Tarsius, 
hedgehog). Sometimes it is (Fig. 23) quite flat (Lepus, Sorex, 
a. 0.). 
A very instructive and in my opinion very archaic case 
among those above-mentioned is that in which the embryonic 
shield remains separated from the overlaying trophoblast by 
a space which arises simultaneously with the growing blasto- 
cyst. This space is from the outset a lenticular or crescentic 
cavity. Its appearance in Erinaceus is elucidated by the 
accompanying diagrams (Figs. 36—38). It represents the 
most typical instance of the manner in which the earliest 
amnion may have arisen as a protective water-cushion between 
the trophoblast and the embryonic shield, and we shall later 
on see that the space within the hedgehog’s amnion is actually 
a later development of this early cavity. In the bat slight 
modifications of this simple arrangement occur, which seem 
to lead on to arrangements as we find them in Tarsius and in 
many Ungulates and Rodents, whereas on the other hand 
Pteropus (Figs. 22, 72), Galeopithecus (Figs. 41, 42), Cavia 
(Figs. 24, 25), monkeys and man (Figs. 59, 40) have developed 
