56 
ON A PLATYPUS EMBRYO, 
could not with certainty be determined at this stage. They 
exhibit no cellular connection with their surroundings and may 
possibly represent the detached neural crest ("Zwischenstrang") of 
which no other representative is present. 
Entoderm. 
The entoderm of the embryonic area presents no very special 
features. It is a single layer of flattened cells, the nuclei of which 
are fairly close. Laterally the cells become more elongated and 
the nuclei consequently are further apart. Here and there in the 
embryonic area these flattened entodermal cells are interspersed 
with large cells distended by the presence in their interior of 
several yolk spheres (c/. fig. 22, vit. ent.). Further out these yolk- 
containing cells become more numerous and eventually form the 
entire inner lining of the blastodermic vesicle. Their structure is 
described later in connection with that of the vesicle. The only 
differentiation of the embryonic entoderm is found in the region 
of the future pharynx. The cells lying just internal to the amnio- 
cardial vesicles have assumed a cubical shape, and form a narrow 
thickened band on each side extending back to the region of the 
heart Anlagen (fig. 5, ent. ph.). These two bands represent the 
pharyngeal entoderm of this region. 
N o to chor d : The notochord in surface view is seen to 
terminate about the middle of the future fore-brain, and sections 
through this region show that the notochord is here represented 
by a thickening of the entoderm forming a median band with 
which the mesoderm is continuous laterally. Many of the cells 
in this anterior region of the notochord exhibit mitotic division 
(fig. 14, nch.). This median thickening as traced back becomes 
more marked and more sharply limited laterally though it is not 
yet distinct from the entoderm. It is in contact above and for 
some distance behind this point with the keel-shaped process of 
the medullary plate. Then just posterior to this the notochord 
becomes distinct as a small rounded mass closely connected with 
the entoderm below and in contact with the keel-shaped process 
of the medullary plate above (fig. 4). Then, from about the 
