BY JAS. P. HILL AND C. J. MARTIN. 
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 slight 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 embryo. According to Selenka these mesoder- 
mal thickenings "gehoren offenbar zur Urwirbelplatte des Kopfes." 
In the Opossum embryo neither the Wolffian 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 view 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 
* Studien liber Entvvick. der Thier. iv. Lief, i Abt. 
D 
