EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 39 
(which then becomes a diplotrophoblast or chorion) thereby 
render the peripheral blastocyst didermic, and may be styled 
parietal or somatic mesoblast; where applied against the 
endodermal vesicle they fall under the category of visceral 
or splanchnic mesoblast (cf. Figs. 45 and 63). 
At the initial spot from whence the proliferation has started 
the ventral mesoblast is naturally more massive than in the 
peripheral, flattened portions, and may here be designated as 
the material out of which the primitive streak and the ventral 
stalk (Haftstiel, Bauchstiel) of the Tarsius embryo takes its 
origin. This stalk-shaped connection between 
embryo and trophoblast is thus present in the 
very earliest stages of development (Fig. 62). 
My conception of the ventral mesoblast in mammals has 
since been adopted by Riickert in his article above cited (’06, 
pp. 1248 and 1251). He compares it with the observations 
hitherto recorded of mesoblast formation in the same region 
in other Amniotes. From its posterior unpaired and median 
point of origin in 'l'arsius it gradually spreads forward right 
and left as the wings of mesoblast are known to do in other 
mammals (“ Mesoderm-sichel”’), and only later this vesicular 
mesoblast (vesicular, because the ccelom is there from the 
beginning, and does not, as far as the extra embryonic ccelom 
is concerned, originate by any ulterior splitting process) also 
appears in front of the embryonic shield, and invades the 
space (cf. Figs. 62 and 63) where the anterior and superior 
entodermal surface of the umbilical sac are yet in close oppo- 
sition with the trophoblast (Hubrecht, ’02, Figs. 48, 5lc, as 
compared to 57a, c). The posterior median portion has 
simultaneously further developed into the incipient, as yet 
extremely delicate Haftstiel which as we saw is there from 
the very beginning, i. e. from the didermic stage downwards. 
