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 alread}^ 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, Tti.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 

 l)ecome 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 al^out two layers of cells connected by 

 cellular bridges (fig. 10, m.s.). 



Lateral trunk mesoderm and central cwlom : 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 l^etween the two (figs. 

 8 and 9). 



Except op]30site the posterior somites the cleavage of the lateral 

 mesoderm does not extend right w^ 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. 



