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 body (PI. ix., tv.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 Wolffian 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 Platj^pus 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. 



