116 FRANKLIN P. REAGAN 



embryo is particularly susceptible to the action of chemicals or 

 to any condition which impairs metabolism. Child (6) has 

 shown that here the rate of metabolism is particularly high. It 

 is very common to find the anterior end greatly retarded or 

 highly abnormal while the posterior end of the embryo may be 

 practically normal. Certainly we may say of the nervous, 

 muscular, skeletal, and alimentary tissues that the characteristics 

 which they exhibit under chemical treatment (see many of 

 Stockard's figures) would give a rather imperfect idea of the 

 normal process. The anterior tissues may be so badly upset 

 as to be scarcely recognizable, while the posterior tissues may 

 approach the normal. The investigator who maintains that it 

 can not be argued that the conditions recorded by Stockard are 

 abnormal 'so far as the blood anlage is concerned' must prove 

 that blood is such a stable tissue as to be able to withstand these 

 ill-effects and develop normally, regardless of the conditions 

 which influence so profoundly the other tissues, especially the 

 anterior tissues. A comparison of Stockard's figures 10 and 11 

 (65) is particularly appropriate at this time. 



Stockard's interpretations are based upon the proposition that 

 if under unfavorable conditions erythrocytes can differentiate in 

 one place in the body, all other regions failing to produce eryth- 

 rocytes under these same conditions are demonstrably powerless 

 to produce erythrocytes under any conditions whatever. Such 

 a view does not permit of different degrees of hematopoetic tend- 

 encies for different regions at a given time; it is claimed that any 

 region which would ever produce blood cells at all must neces- 

 sarily produce them under the given abnormal condition. In 

 other words, if erythrocytes form at all they must necessarily 

 form in all places capable of producing them at any time under 

 normal conditions. From the fact that endothelium grown in 

 an alcoholic environment was able to produce no erythrocytes, 

 Stockard (65, p. 276) maintains that ''one is warranted in mak- 

 ing the bold assertion that the endothelial lining of the heart 

 and aorta is perfectly incapable of giving rise to any type of 

 blood-cell." As a matter of fact it has never been maintained 

 that endothelium is an important source of erythrocytes as com- 



