196 PROFESSOR MARSHALL AND EDWARD J. BLES. 



development of the ear and other organs. The mode of formation 

 of the mesoblast is also in many respects very far removed from the 

 typical Vertebrate plan. We are inclined, therefore, to regard the 

 cardiac endothelium, not as consisting of hypoblast cells, but of 

 mesoblast cells, formed from the hypoblast in a somewhat unusual 

 manner, and at a place where we have shown above there are no 

 other mesoblast cells. 



The statement that has been recently made* that the notochord 

 in the frog is of mesoblastic and not of hypoblastic origin, is, we 

 believe, misleading, for the same reason, and due to a failure to 

 appreciate the great modification which the early development of 

 the frog has undergone. We have paid some attention to the point, 

 and find the notochord at its posterior end to be clearly and distinctly 

 hypoblastic, and in its anterior part to be formed from a mass of 

 cells concerning which it is impossible to determine whether they 

 are to be spoken of as hypoblastic or mesoblastic. 



II. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN 4| mm. TADPOLES. 



In describing the later phases in the development of the heart, and 

 the mode of formation of the principal blood vessels, we propose to 

 take a series of stages selected somewhat arbitrarily, but which we 

 have found to represent in a convenient manner the principal steps 

 in the history. We have defined these stages by the length of the 

 tadpole ; but as there is a considerable amount of variability in this 

 respect, we have in each case given a brief account of the external 

 features, and of the leading points of internal structure, which we 

 trust will render the determination of our stages a comparatively 

 easy matter. 



Our figures are in almost all cases highly diagrammatic. They have 

 been constructed in order to facilitate as far as possible comparison 

 of the several stages with one another. In each figure many impor- 

 tant organs are entirely omitted, but the parts which are shown have 

 been drawn with great care, and their proportions and relative 

 positions are shown accurately. 



* Schultze, O., " Die Entwickelung der Keimblatter und der Chorda dorsalis von 

 Rana fusca," " Zeitschrift fur wissenschaftliche Zoologie," Bd. 47, 1888, pp. 325-352. 



