40 FRANKLIN P. REAGAN 



Part II 



THK ORIGIN OF BLOOD CORPUSCLES IN TELEOST EMBRYOS 



I. A study of cnihryos without circulation 90 



1. Hybrid material 91 



2. Chemically treated embryos 91 



3. Hematopoesis in the liver 105 • 



4. Ova subjected to low temperatures 107 



5. Cardiectomized embryos 107 



6. Transplanted cephalic meroplasts 112 



II. General Considerations 



1. Does abnormal treatment obscure or alter the normal process?. . . 115 



2. Are different regions unequally susceptible to chemical action?. . . 116 



3. Does the fact of differentiation coincide with the polyphyletic 



theory? 121 



4. Tissue specificity 123 



5. Mesenchyme and preformation 125 



III. Summary 126 



IV. Literature cited 127 



INTRODUCTION 



The following contribution represents a resume of three years 

 of experimental study in which an attempt has been made to 

 analyze certain of the processes involved in the genesis of some 

 of the vascular tissues. For constant encouragement and most 

 helpful criticism I wish to acknowledge my deep indebtedness to 

 Prof. Charles F. W. McClure, under whose guidance the work 

 was undertaken. I am especially grateful for the generosity 

 with which he has furnished me with necessary funds and ap- 

 paratus for the undertaking of the problem at hand. I have 

 also been so fortunate as to have recourse to the valuable counsel 

 of Prof. E. G. Conklin, whose advice has been of great help to 

 me. 



PART I. THE ORIGIN OF ENDOTHELIUM 



In many respects the development of the vascular tissue is 

 unique. Its investigation is attended with certain peculiar 

 difficulties which have hindered the solution even of some of 

 the least involved problems in this field of development. Within 



