58 
ON 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 medullary 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 coelomic 
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. They 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 medullary plate (tigs. 1 and 
15, pra.). 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 medullary plate, of scattered 
Human Embyology, 1892, p. 198. 
