370 Florence Peebles. 



again used the method described in my earlier work/ A small 

 window was made in the shell just above the blastoderm, and the 

 operation performed, after which the opening was closed by a 

 piece of shell, sealed with strips of the shell membrane. All in- 

 struments used in the experiments were carefully sterilized, and 

 the shells of freshly opened eggs used for closing the windows. 

 The loss of eggs through infection was small. In most of the 

 experiments one egg in each set was opened and then sealed again, 

 without operating upon it, in order to have a check with which to 

 compare the eggs upon which experiments were made. In this 

 way it was possible to determine roughly, after further incubation, 

 whether abnormalities were due to opening the egg or to the 

 operation performed upon the blastoderm. In general it was 

 found that the development of eggs in which windows were made 

 was delayed about two to four hours. 



I. THE LOCATION OF THE EMBRYO IN THE MATERIAL OF THE 

 UNINCUBATED BLASTODERM 



In 1896, Assheton^ described some experiments that he made 

 on the unincubated blastoderm of the chick. Sable hairs were 

 inserted at various points and their position determined after 

 periods of incubation varying from eighteen to forty hours. 

 Assheton proved that Duval's^ theory of the formation of the 

 primitive streak is incorrect, that instead of forming by the con- 

 crescence of the posterior margin of the blastoderm, the primitive 

 streak appears in the region of the unincubated blastoderm which 

 lies between the center and the posterior margin of the area 

 pellucida. I have repeated Assheton's experiments, making the 

 injuries with a hot needle instead of a hair, without removing the 

 egg from the shell. The results agree with those of Assheton, as 

 the following experiments show: 



'Peebles, Florence. Some Experiments on the Primitive Streak of the Chick. 

 Archiv. fur Entwickelungsmech. der Organismen. VII Band. 1898. 



^Assheton, R. An Experimental Examination into the Growth of the Blastoderm 

 of the Chick. Proceedings of the Royal Soc, Vol. 63, 1896. 



^Duval. De la Formation du Blastoderme dans I'Oeuf d'Oiseau. Annales des 

 Sciences Naturelles, Zoologie, Vol. 18. 



