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. e.7it.). Further out these yolk- 

 containing cells l:)ecome more numerous and eventually form the 

 entire inner lining of the blastodermic vesicle. Their structure is 

 described later in cc^niection 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 (tig. 5, pmI. ph.). These two bands represent the 

 pharyngeal entoderm of this region. 



N oto chord : 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 



