174 CHARLES R. STOCKARD 



I 



The embryos in which there has been no circulation of the blood 

 form the blood islands from the wandering cells on the yolk-sac, 

 and the constituent elements of these islands differentiate per- 

 fectly and may maintain their red color for many days. Yet 

 they never leave the locality in which they have differentiated. 

 The fully formed red blood corpuscles have little if any power 

 of migrating. When the observer can be positive that the blood 

 has never circulated, and this requires very consistent watching, 

 the islands of the yolk are always limited to certain regions and 

 never occur so far anteriorly on the ventral surface of the yolk 

 as to reach the venous end of the heart. 



5. On the yolk-sac of Fundulus embryos one thus finds four 

 distinctly different products differentiating from the apparently 

 similar wandering mesenchymal cells. The environment in 

 which the four types differentiate is identical as far as is possible 

 to determine, and the .only explanation of their various modes 

 of differentiation is that the original mesenchymal cells that 

 wandered out were already of four potentially different classes. 

 These differences in potentiality within the early cells gave rise to 

 the four different directions of cytomorphosis in one and the 

 same environment. The four resulting types of cells are then in 

 an embryological sense derived from different mesenchymal 

 anlagen. 



(Only those titles not given in the previous paper) 

 AGASSIZ, L. AND WHITMAN, C. O. 1884 On the development of some pelagic 



fish eggs. Proc. Amer. Acad. Arts and Sci., xx. 

 BREMER, J. L. 1914 The earliest blood-vessels in man. Amer. Journ. Anat., 



16. 

 EVANS, H. M. 1909 On the development of the aortae, cardinal and umbilical 



veins and the other blood vessels of vertebrate embryos from 



capillaries. Anat. Record, 3. 

 RYDER, J. A. 1884 A contribution to the embryology of osseous fishes. Report 



U. S. Fish Comm. for 1882, Washington. 



1887 On the development of osseous fishes. Report U. S. Fish Comm. 



for 1885, Washington. 

 THOMA, R. 1893 TJntersuchungen ueber die Histogenese und Histomechanik 



des Gefasssystems. Enke, Stuttgart. 



1896 Text-book of general pathology and pathological anatomy. 



Translated by Bruce, London. 



