POSITION OF THE PRIMITIVE STREAK. '213 



which I used Kopsch's method of injuring certain regions by the 

 introduction of electrodes instead of cauterizing with a hot 

 , needle. I was, not so successful with this method as I have been 

 with the hot needle. I repeated my own experiments in which 

 the anterior end of the primitive streak of eighteen hours had 

 been killed, and obtained the same result as before ; the injury 

 appeared after twenty-four to forty-eight hours in the region of 

 the anterior somites. Kopsch did not injure the anterior end of 

 the primitive streak of an embryo younger than twenty-four 

 hours. He found that an injury made at the anterior end of the 

 primitive streak at this time appears after incubation in the brain, 

 greatly disturbing the development of that region. From this 

 he concludes that the anterior end of the primitive streak repre- 

 sents the chordal region of the head. I have found that an in- 

 jury made in the area pellucida immediately /// front of the 

 primitive streak of eighteen hours, appears in this region of 

 the head. It seems reasonable, therefore, to conclude that 

 Kopsch has injured the head process instead of the primitive 

 streak, for by the twenty-fourth hour the former is fully de- 

 veloped. 



I can not understand how Kopsch is able to locate the anterior 

 end of the primitive streak in a twenty-four-hour embryo, for 

 eggs incubated in the laboratory at Bryn Mawr, and in my own 

 laboratory in Baltimore, at twenty-four hours show the medullary 

 folds, the first pair of somites, and the notochord ; moreover, in 

 the living egg it is almost impossible to determine where the 

 primitive streak ends and the head process begins. After the 

 twentieth hour I have found it impossible, unless development 

 has been delayed, to locate the anterior end of the primitive 

 streak. 



I have repeated Kopsch's experiment, and have intentionally 

 injured the distal end of the head process. At the end of twenty- 

 four to forty-eight hours the injured region has been found in 

 the brain, in a position corresponding to that indicated by 

 Kopsch as the result of supposed injury to the anterior end of 

 the primitive streak. If Kopsch had experimented on an em- 

 bryo of the age indicated by his figures for sixteen and a half 



