BY JAS. P. HILL AND C. J. MARTIN. 69 



33). The greater part of the chromatin of tlie nucleus is con- 

 tracted into a star-shaped mass in the centre, while smaller 

 pai'ticles of chromatin occui- 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 embr3'o. 

 The lateral extension of the mesoderm diminishes gradually as 

 one proceeds forwards, so that in the region of the heart Anlagen 

 it extends onl}^ 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 Ijeyond the 

 point where the amnio-cardial vesicles converge to limit the pro- 

 amnion, mesoderm is entirely absent. 



Beyond the coelom there extends out a layer of flattened meso- 

 dermal cells ])etween which and the vitelline entoderm is a layer 

 of numerous rounded vasifactive cells (fig. 30, cas. 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, 

 mfi,-<.). 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 

 8auropsida and the typical mammalian blastodermic vesicle. 



In the Sauropsida it is only after most of the yolk has Ijeen 

 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 l)lastodermic 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 embrj^o, 

 instead of overlying a mass of unsegmented 3'^olk, lies on the 

 surface of a two-layered vesicle containing fluid, which is only 

 distinguishable from a typical mammalian Ijlastodermic vesicle 

 through the fact that instead of having a \^olk-free entoderm, it 

 possesses an entoderm composed of large yolk-containing cells. 



