4A A. A. W. HUBRECH'. 
notogenesis), whereas the derivates of the ventral mesoblast 
find employment in the construction of the posterior and 
postero-ventral portions of the embryo. 
It will here suffice to state that the extra embryonic 
ccelom which is present in Tarsius (and undoubtedly in 
monkeys and man) in the ventral mesoblast at so very early 
a phase (extending as was described on p. 38 behind and 
below the endodermic vesicle and the ectodermal shield) 
first makes its appearance in the other mammals at a later 
period, but exactly in the same position, viz. behind the 
embryonic shield (Figs. 43, 61, and 100). From there it 
eradually extends in crescent shape right and left along the 
hind margin of the embryonic shield. This ccelom—con- 
siderably less spacious and less precocious than that of the 
Primates—is fully homologous to it, both as regards the place 
where it is found, the cell material in which it appears, and 
the relation in which it stands to the ccelom of the somites 
and lateral plates, as will be described later on. Bonnet’s 
(82, 789), Keibel’s (93), and my own (’02) observations on 
the appearance of this crescent-shaped ccelom are in perfect 
agreement with each other, as also those concerning the 
fact that this ventral coelom only later fuses with the intra- 
embryonic ccelom (Keibel, 93, figs. 39 and 402; Hubrecht, 
02, fig. 77d). The pericardial coelom arises independently 
along the front border of the embryonic shield, and will also 
be more fully discussed later on (Hubrecht, ’02, p. 87, 
figs. 70, 73). 
Summarising what we have here rapidly sketched we may 
agree to have seen that instead of a homogeneous median 
germ layer, instead of a mesoderm which has the same mor- 
phological importance as the two primary germ layers and ori- 
ginates from the coalescing lips of a blastopore, we find at least 
three foci of cell-activity in those primary germ layers. The 
appearance of these foci marks the end of the didermic stage 
of the blastocyst. In consequence of processes of prolifera- 
tion and rapid mitosis there is started from these three 
centres a host of new cells, which, together, spread between 
