BY JAS. P. HILL AND C. J. MARTIX. 49 



In fact we are not acquainted with any embryo which reaches 

 the dimensions mentioned above and is possessed of such a number 

 (17 pairs) of somites and which yet remains, with the exception 

 of a sHght head fold, absolutely flat. 



Selenka's"^ figure (fig. 1 Taf. xxi.) of a three days' Didelphys 

 embryo does however present considerable points of resemblance to 

 the Platypus embryo under consideration, though it is very much 

 smaller (4'5 mm.) and possesses only fourteen somites. 



In both embryos the medullary plates are practically flat, 

 double heart Anlagen are present, and head fold formed. The 

 appearance of the anterior end of the medullary plate of the three 

 days' Opossum closely resembles that of the Platypus embryo; and 

 further, in the region of the future mid-brain the same lateral 

 mesodermal thickenings occur [vide Selenka's fig. 4, Taf. xx. y.) as 

 we have described above, though in the Opossum they are not so 

 marked as in our embr3^o. According to Selenka these mesoder- 

 mal thickenings "gehoren offenbar zur Urwirbelplatte des Kopfes." 



In the Opossum embryo neither the Wolflian body nor the 

 auditory plates are indicated, nor are there any neuromeres 

 described. It however seems highly probable to us that the 

 structures situated in the region of the hind-brain which Selenka 

 regards as the anterior five somites are in reality not somites at 

 all but true neuromeres. A comparison of his fig. 4, Taf. xx., 

 which represents a slightly younger embryo, with the above- 

 mentioned figure renders this view still more likely. In his 

 figures the structures regarded as the anterior five somites extend 

 inwards from the edges of the medullary plate to within a short 

 distance of the mesial line, and in surface ^iew appear related 

 essentially as are the neuromeres in our embryo; while the 

 remaining somites of the trunk, instead of ending on a level with 

 the edges of the medullary plate, extend out beyond them. 



Further, his transverse section (fig. 3, Taf. xxi.) through the 

 region of the hind-brain and passing through one of these 

 supposed somites shows no mesodermal differentiation which could 



^■" Studieii iiber Entwick. der Thier. iv. Lief, i Abt. 

 D 



