ORIGIN OF BLOOD AND ENDOTHELIUM . 83 



The weight of evidence at the present time is then in favor 

 of the earlier notion of Ziegler. The blood vascular system if 

 it is associated with, or phylogenetically derived from any other 

 body cavity, that cavity is really the primary body cavity or 

 embryologically the blastocoel. 



4. Haematopoesis, the monophyletic and polyphyletic views, etc. 



The experiments recorded above are of particular value in the 

 solution of that very complex problem, the origin and relation- 

 ship of the different types of blood corpuscles. We may here 

 then briefly consider the evidence they furnish in connection 

 with the various theories and points of view recently advanced 

 in explanation of the origin of blood cells. 



The vertebrate animals present two entirely different types 

 of cells floating in their blood fluid. The white blood corpuscles 

 are cells of primitive type and are not only found within the ves- 

 sels but they also wander through the interstitial spaces of all the 

 tissues of the body. These wandering white blood cells, amoebo- 

 cytes, are almost universally distributed throughout the animal 

 kingdom being found in all the invertebrate groups above the 

 one or two very lowest as well as in all the vertebrate classes. 

 In no animal do these cells contain haemoglobin, haemocyanin 

 or any compound that would particularly qualify them as oxy- 

 gen carriers, or give to them any function as an organ of respira- 

 tion. These white blood cells found outside of the blood currents 

 as well as in the blood are to be looked upon as cells which are 

 not particularly associated with any specific blood function. 

 They merely find the blood current a ready or rapid means of 

 being carried from place to place within the body. 



The red blood corpuscles, erythrocytes, are in contrast to the 

 white cells a very highly specialized type of cell and specifically 

 a blood cell. In fact, this is one of the most specialized cells 

 within the body. In mammals, for example, it is specialized 

 to such a degree that its functional perfection is actually accom- 

 panied by the loss of its nucleus and necessarily, therefore, the 

 loss of its own future existence after a short period of time. 



