60 
ON A PLATYPUS EMBRYO, 
plate lying close below the ectoderm immediately external to the 
edge of the medullary plate. Beneath it there exist looser stellate 
cells which are continuous with it at both ends (fig. 7, mes. ax.). 
Mesodermic somites : The appearance of the seventeen pairs of 
somites as seen in surface view has already been described. With 
the exception of the first three and the last three the somites are 
practically identical. In transverse sections (figs. 8 and 9, m.s.) 
they present an oblong form compressed dorso-ventrally and 
extend some distance beneath the medullary plate. They possess 
in their whole breadth very distinct myotomic cavities bounded 
by dorsal and ventral walls composed of somewhat stellate cells. 
The cavity is sometimes interrupted by strands of cells passing 
between the two walls. 
The ventral walls of the first three pairs of somites have 
become converted into stellate mesenchyme cells, and in the first 
at least the myotomic cavity is no longer distinguishable (fig. 7). 
Their dorsal walls form a somewhat arched plate of closely 
compacted cells. 
The last three pairs of somites do not possess well marked 
cavities. They consist of about two layers of cells connected by 
cellular bridges (fig. 10, m.s.). 
Lateral trunk mesoderm and ventral covlom : The lateral meso- 
derm appears directly continuous with the first three somites, 
without any intermediate cell mass, while from the 4th onwards 
a distinct intermediate cell mass is present between the two (figs. 
8 and 9). 
Except opposite the posterior somites the cleavage of the lateral 
mesoderm does not extend right up to the somites, the ccelom 
only appearing some distance out. The splanchnic layer is only 
one cell thick, while the main portion of the mesoderm continues 
out as a thick somatic layer. This gradually thins as it passes 
out, becomes reduced to a single layer of cells, and ultimately 
fuses with the splanchnic layer to constitute a single mesodermal 
layer marginally. 
