170 EMBRYOLOGY. 



by the greater ease with which all its processes of development are 

 understood, has failed us in this question. For HATSCHEK, who 

 knows the development of Amphioxus better than any one else, de- 

 signates the blood-vessels as the only system of organs concerning 

 which he was unable to arrive at a clear understanding. 



Consequently in the field now to be examined there are many 

 views and observations which in part stand in the most direct 

 antagonism to each other. To give a comprehensive review of them 

 is not possible without the greatest fulness, which would be contrary 

 to the plan of this Text-book; I therefore limit myself, first, to 

 giving a survey of the various possibilities by which the origin of 

 the vessels and the blood might take place, and, secondly, to present- 

 ing a series of observations which have been made on Selachians, 

 Birds, and Mammals ; still it is always to be kept in mind that 

 much remains doubtful here, and that coming years may bring about 

 many a change in our interpretations. 



According to one view, the vascular cavities are developed out of 

 fissure-like spaces between the germ-layers which remain unoccupied at 

 the time the fundament of the mesenchyme is produced. These cavities 

 acquire a boundary in this way : the neighboring mesenchyme-cells 

 begin to penetrate into them, and then unite into a vascular endo- 

 thelium. " The system of blood-vessels and that of lymphatic vessels," 

 observes ZIEGLER, " are produced in their first fundaments from 

 remnants of the primary body-cavity (the space between the primary 

 germ-layers), which at the general distribution of the formative 

 tissue (mesenchyma) remain behind as vessels, lacunae, or interstices, 

 and are enclosed by that tissue and incorporated in it." The formed 

 elements [corpuscles] arise at separate places in the blood-courses 

 by the growth and detachment of mesenchymatic cells. 



According to another view, the vessels are constructed in this 

 manner : cells in the mesenchymatic tissue arrange themselves in 

 rows, and these cell-cords become hollowed out ; thereby the more 

 superficial cells furnish the endothelial wall, whereas the remaining 

 cells become blood-corpuscles. The blood-vessels are therefore nothing 

 else than cavities which have been secondarily produced in the 

 mesenchymatic tissues by means of their own cells. Both views 

 agree in this, that they cause the group of sustentative substances 

 to be brought into genetic connection with the blood, and the latter 

 f o figure as a product of the metamorphosis of the mesenchyma. 



Moreover, both views may present variations in the details, 

 according as they ascribe to the mesenchyme a different origin and 



