17G KM BRYOLOGY. 



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- 

 utes 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, ami 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 

 observes ZIEGLER, " are produced in their first fundaments from 

 remnants of the primary body-en vity (the space between the prin 

 germ-layers), which at tli< distribution of the formative 



tissue (mesenchyma) remain behind as vessels, lacuna?, or in 

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

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

 by the growth and detachment of mesenchymatic cc-ll>. 



According to another view, the vessels are constructed in this 



manner: n-lU in the mesenchymatic tissue airsn^' them>elve> in 



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



>uj>erfieia] cells furnish the endothelial wall, whereas the remaining 



liecome blood-corpuscles. The blood-vessels are therefore nothing 



else than cavities which have been secondarily produced in the 



mesenchymatic tissues by means of thi ir own cells. Both vie\\> 



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



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



product of the metamorphosis of the mesenchyma. 



Moreover, both views may present variations in the details, 

 according as they ascribe to the me&enchyme a different origin ai <1 



