58 ox A PLATYPUS EMBRYO, 



A transverse section through the middle region of the head 

 plate is shown in fig. 4. Here, below the medullary plate, the 

 mesoderm exists in the form of scattered stellate cells, while 

 laterally to the outer edges of the medullar}^ plate the cells are 

 more numerous and more closely packed, especially immediately 

 below the ectoderm and at the outer rim of the head plate. This 

 thickened rim marks the outer contour of each plate as seen in 

 surface view. Beyond this rim the mesoderm is divided by the 

 development of a ccelom into two layers, an upper thin layer of 

 flattened somatic mesoderm cells and a lower thicker layer of some- 

 what cubical splanchnic mesoderm cells. The narrow ccelomic 

 spaces thus enclosed when traced posteriorly are found to be the 

 most anterior parts of the body cavity, and for them we adopt 

 Minot's* term amnio-cardial vesicles. 



The amnio-cardial vesicles extend forwards as two horns beyond 

 the anterior end of the embryo. The}" converge towards the 

 median line without, however, uniting, and practically limit a 

 small area (the proamnion) in which mesoderm is absent. The 

 proamnion is of very small extent and lies immediately in front 

 of and below the anterior end of the medullar}^ plate (figs. 1 and 

 15, 2jra.). Behind the posterior limits of the head plates the 

 mesoderm becomes reduced to a very thin layer, and is absent 

 altogether over a limited area just anterior to the outermost 

 portion of each auditory plate (fig. 5), and here ectoderm and 

 entoderm come into contact as in the proamnial region. The thin 

 areas of mesoderm behind the head plates are very obvious in the 

 photo-micrograph as the lighter areas in the middle of which the 

 auditory plates are situated. These thin areas are wholly confined 

 to the forward continuations of the protovertebral zones of meso- 

 derm. Externally to the thin areas are the forward extensions 

 of the lateral trunk zones of mesoderm, along the outer edges of 

 which are situated the symmetrical heart Anlagen. 



The mesoderm in the hind-brain region is entirely destitute of 

 segmentation. It consists, below the medullar}^ j^^^^^j of scattered 



* Human Embyology, 1S92, p. 198. 



