48 
ON A PLATYPUS EMBRYO, 
mesoderm, nearly up to the first somite. They are thus situated 
in greater part opposite the hind-brain region. 
In the trunk region, on either side of and extending below the 
medullary plate there are seventeen pairs of somites : the first pair 
situated relatively far back with regard to the auditory plates, 
and separated by a considerable space from the last neuromere. 
The anterior somites are square in shape with their borders at 
right angles to the axial line. They become successively broader 
and shorter towards the posterior end of the embryo, where they 
are placed obliquely to its long axis. 
On each side just external to the outer edges of the somites 
(with the exception of the first three) and between them and the 
lateral zone of mesoderm there occurs a narrow intermediate 
zone containing the Anlage of the Wolffian bod}^ (PI. ix., w.b.). 
Beginning as a faint line opposite the fourth somite, it becomes 
more distinct opposite the seventh, and from thence backwards as 
far as the fifteenth somite it exhibits an irregular linear thicken- 
ing. Over this posterior part of its course the Wolfiian duct 
occurs as a distinct structure. Behind the fifteenth somite the 
intermediate cell mass extends backwards as a narrow strip some- 
what beyond the last somite. 
Caldwell* compared the Platypus embryo from an egg just laid 
to a chick of about 36 hrs., but beyond the number of somites 
being about the same in both, there is hardly any other point of 
comparison. In a chick of this age the medullary groove is 
closed right down to the sinus rhomboidalis, the head is covered 
by the amnion, the three cerebral vesicles and the optic vesicles 
are well developed, the heart is formed and already bent, the 
vascular area differentiated and the blood circulating; whereas in 
the Platypus embryo at about the time of laying, the medullary 
plate is practically flat, vascular area and amnion are non-existent, 
while the heart is represented by two lateral Anlagen at the 
periphery of the anterior extensions of the lateral plates of 
mesoderm. 
Proc. Roy. Soc. N.S.W. Vol. xviii. 1884, p. 120. 
