372 Florence Peebles. 



as opened above the blastoderm. The air chamber, which Hes in 

 the blunt end of the shell, is at the left, and the pointed end at the 

 right. The chalazae extend on each side of the yolk in the long 

 axis of the shell. In this position the blastoderm may be divided 

 into right and left halves, the arrow a-b indicating the median 

 plane of the bi-laterally symmetrical embryo. We then speak of 

 the region c as the anterior border, and d as the posterior border 

 of the blastoderm. 



Assheton, in another series of experiments, has made two 

 injuries in the unincubated blastoderm (Text-fig. 3) one in the 

 center (x) and the other in the posterior border [y) of the area 

 pellucida. He found that the primitive streak appeared later 

 between these two injuries, and he concluded from this that the 

 point (jy) marks the posterior end of the embryo. 



In order to distinguish the region of the primitive streak from 

 the rest of the area pellucida, I shall call it the radius x-y. This 

 radius with the corresponding one anterior to the center make the 

 diameter which represents the median longitudinal axis of the 

 embryo. In order to determine the constancy of the occurrence 

 of the embryo in this position I have kept the record of 100 eggs. 

 The eggs were taken from the nest on the same day that they were 

 laid. They were placed in the same position in a basket from 

 which they were transferred to the incubator. After incubation 

 for eighteen to forty-eight hours the embryo in every fertile egg, 

 with the exception of two, was found in the median line (Text-fig. 

 2 a-b). The two exceptions are shown in Text-figs. 4 and 5. 

 The first embryo (Text-fig. 4) was incubated eighteen hours. 

 At the end of this time the primitive streak had formed, but in- 

 stead of lying on the radius x-y it extended from the center to the 

 right side of the blastoderm and was bent towards the posterior 

 margin. The second egg (Text-fig. 5) was incubated for a period 

 of twenty-eight hours. The normal embryo lay at right angles to 

 the line a-b. In both of these eggs the chalazae were found in 

 abnormal positions, and the yolk membrane was wrinkled in many 

 places, showing that the yolk had been abnormally twisted in its 

 passage through the oviduct. 



After I had discovered that the position of the normal embryo, 

 when undisturbed by twisting or shaking, is constant, I deter- 



