66 ON A PLATYPUS EMBRYO, 



In its topographical relations the Wolffian duct in Platypus 

 agrees with the conditions described by Meyer"^ in man, where 

 according to him the proximal part of the duct leads back from 

 the mesoderm while its distal portion is connected with the 

 ectoderm. Both Meyer and Martin agree in describing the 

 proximal part of the duct as mesodermal in origin, but as to this 

 we are not in a j)Osition to speak with certainty. However, in 

 Platypus the proximal part of the united Anlagen of the Wolffian 

 duct and tubules is related essentially as Martin describes for the 

 corresponding portion in the rabbit, and it might well be that as 

 in that animal the proximal portion of the Wolffian duct Anlage 

 arises from the intermediate cell mass in common with the 

 Anlagen of the Wolffian tubules. 



Vascu/ar System. 



Heart Anlagen: The symmetrically placed heart Anlagen have 

 already been descril)ed in surface view as situated in greater part 

 opposite the hind-brain region. Sections, however, show that 

 their anterior ends extend somewhat beyond the posterior limits 

 of the head plates of mesoblast, and we may therefore look upon 

 these head plates as the regions in which the future aortic arches 

 will be developed. 



The heart Anlagen at this stage in Platypus are essentially 

 similar to those of a rabbit of about nine days. 



As the amnio-cardial vesicles are traced from their anterior 

 ends backwards, they gradually increase in lateral extent, and at 

 the same time towards the posterior limits of the head plates the 

 thick splanchnic layer of mesoderm separates from the entoderm. 

 In the space thus formed on each side, rounded vasifactive cells 

 appear. Posteriorly these vasifactive cells have formed the 

 vascular endothelium of the anterior cardiac region, and this lies 

 in the gutter-like groove— open towards the entoderm — formed 

 by the inbulging of the thick splanchnic mesoderm into the amnio- 

 cardial ctelom. Each heart Anlage consists anteriorly of several 



* Arch. f. Mikr. Anat. Bd. 36, 1890. 



