108 FRANKLIN P. REAGAN 



to gravity, and it is possible to obtain embryos from which body- 

 movements can not be elicited. The method has other advan- 

 tages. The only unfavorable chemical effect is produced by the 

 lack of tissue nourishment and tissue respiration, and the accumu- 

 lation of katabolic materials together with the possibility of in- 

 fection. Unfortunately these are not always insignificant. The 

 rate of mortality from such operations is very high indeed, prob- 

 ably because of infection. But once an embryo is able to re- 

 cover somewhat from the operation it furnishes valuable mate- 

 rial for study. In chemical treatment the specific ill-effects of 

 the chemical used are to be added to inanition, the lack of tissue- 

 respiration and the accumulation of katabohc products. Fur- 

 thermore in case of cardiectomy the embryos may in some in- 

 stances be allowed to develop normally for seventy hours before 

 operation is necessary to prevent circulation, while in chemical 

 treatment the entire development for the first one to three days 

 must take place in a very abnormal environment. But the one 

 characteristic in common with the results of both sorts of treat- 

 ment is that the anterior region of the body always suffers the 

 most. It seems practically impossible to obtain cardiectomized 

 embryos whose anterior tissues are perfectly normal, though their 

 posterior regions are generally little injured. In some cases a 

 very small portion of the head was severed from the rest of the 

 body and from the heart; it was hoped that the mesenchyme in 

 such meroplasts would be able to form erythrocytes. It was 

 found, however, that these head-fragments would not increase in 

 size. Thus it was found later that the diameter of such frag- 

 ments was always much smaller than that of the remainder of 

 the body (figs. 84 and 85). The mesenchyme in these small 

 fragments always became very closely packed, so that it is not 

 surprising that blood-cells were unable to develop under these 

 conditions. 



There is reason to believe that in some instances the condi- 

 tions obtained by cardiectomy may represent more faithfully the 

 normal process than can be obtained by chemical treatment. 

 The former method is capable of producing stages more advanced 

 in differentiation and growth than one can obtain by chemical 



