92 FRANKLIN P. REAGAN 



on embryos treated with weak solutions of acetone, butyric acid 

 or potassium cyanide. Generally the ova were placed in these 

 solutions for twenty-four hours or less, and then reared in run- 

 ning sea-water. The results obtained demonstrate that eryth- 

 rocytes form in the intermediate cell mass and on the posterior 

 yolk; in addition to this, they indicate that erythrocytes may 

 form in any of those locations in which Stockard claims they 

 never form in embryos without circulation. Some of the con- 

 ditions about to be described have been previously reported 

 (53). 



It is a striking fact that under chemical treatment the em- 

 bryos tend to exhibit abnormalities. Still more striking is the 

 fact that all the tissues in the anterior end seem to be the most 

 seriously affected. It is rare indeed that the circulation can be 

 inhibited without injury to the anterior end of the body. The 

 condition of oedema in the pericardium produced by chemical 

 treatment swells the pericardium, thus stretching the heart until 

 it becomes solid, or until it is unable to propel the blood. A 

 treatment sufficiently rigorous to render the pericardial cavity 

 oedematous is generally sufficient to create marked disturbance 

 in the anterior tissues. By studying a very large number of 

 embryos, however, one is able to find a few cases in which the 

 circulation has been inhibited and yet at the same time the an- 

 terior region bears some resemblance to the normal condition. 

 If we concentrate our study on such embryos as these, we ar- 

 rive at very different results from those of the observer who con- 

 centrates on the majority of embryos whose anterior ends have 

 suffered specifically from the abnormal treatment. Whether the 

 anterior end is injured or remains normal, the posterior end of 

 the chemically treated embryo generally is very little deranged. 

 A lethal treatment is often without serious effect on the posterior 

 region. The explanation of this phenomenon is very difficult. 

 It is merely one of the axioms of development which we must 

 recognize. Child (6) deals with this phenomenon under the 

 term 'axial gradient.' 



The vascular tissues on the teleost yolk-sac are formed largely 

 from mesenchyme cells which migrate there from the posterior 



