BY JAS. P. HILL AND C. J. MARTIN. 
69 
33). The greater part of the chromatin of the nucleus is con- 
tracted into a star-shaped mass in the centre, while smaller 
particles of chromatin occur sparsely around this. 
The mesoderm extends round from a quarter to a half of the 
circumference of the vesicle in the posterior region of the embryo. 
The lateral extension of the mesoderm diminishes gradually as 
one proceeds forwards, so that in the region of the heart Anlagen 
it extends only a short distance laterally to them, while in the 
region of the head plates of mesoderm the amnio-cardial vesicles 
form its outermost limit. In front of the embryo beyond the 
point where the amnio-cardial vesicles converge to limit the pro- 
amnion, mesoderm is entirely absent. 
Beyond the ccelom there extends out a layer of flattened meso- 
dermal cells between which and the vitelline entoderm is a layer 
of numerous rounded vasifactive cells (fig. 30, ccts. c). Further- 
out these two mesodermal layers are continued into a layer of some- 
what spindle-shaped cells with large rounded nuclei which forms 
the outermost portion of the extra-embryonic mesoderm (fig. 31, 
niest<.). It is from the relatively very early great lateral extension 
of the mesoderm and from the presence of a very distinct yolk- 
containing entoderm that we regard the vesicle of the Platypus 
embryo of this stage as transitional between the yolk sac of 
Sauropsida and the typical mammalian blastodermic vesicle. 
In the Sauropsida it is only after most of the yolk has been 
absorbed that the yolk sac is completely lined by discrete ento- 
dermal cells; in the higher mammalia, on the other hand, in the 
absence of yolk, the entoderm — the homologue of the yolk mass 
of Sauropsida — is very early able to completely enclose the cavity 
of the blastodermic vesicle — the homologue of the yolk sac cavity 
of Sauropsida. The ovarian ovum of the Platypus is as is well 
known a typical yolk-laden egg, yet at this stage the embryo, 
instead of overlying a mass of unsegmented yolk, lies on the 
surface of a two-layered vesicle containing fluid, which is only 
distinguishable from a typical mammalian blastodermic vesicle 
through the fact that instead of having a yolk-free entoderm, it 
possesses an entoderm composed of large yolk-containing cells. 
