A PRELIMINARY NOTE ON THE POSITION OF 

 THE PRIMITIVE STREAK, AND ITS RELA- 

 TION TO THE EMBRYO OF THE CHICK. 



FLORENCE PEEBLES. 



At the end of the eighth to tenth hour of incubation of the 

 hen's egg, the primitive streak begins to appear as an opaque 

 line extending from the inner edge of the area pellucida to the 

 center of the blastoderm. Between the tenth and eighteenth 

 hours the primitive streak extends in length, until it covers a 

 distance equal to two-thirds of the entire length of the area pel-* 

 lucida. While this increase in length is taking place the primi- 

 tive groove forms, and at about the twentieth hour the head 

 process is visible as a thickened line extending anteriorly from 

 the primitive streak. I have endeavored to determine, experi- 

 mentally, the direction of growth, and the exact position of the 

 primitive streak in relation to the later embryo. 



Assheton concluded from the results of his experiments that 

 the central point of the area pellucida, before it becomes pear- 

 shaped, represents the anterior end of the primitive streak, and 

 that the latter develops "from the portion of the unincubated 

 blastoderm which lies between the center of the blastoderm and 

 the posterior margin of the area pellucida." Assheton does not 

 mention the number of hours that the eggs were incubated, so 

 that we do not know whether this central point represents the 

 anterior end of the young primitive streak, or the point from 

 which, later, the head process extends. His figures indicate, how- 

 ever, that the central point of the area pellucida represents the 

 anterior end of the completely formed primitive streak of the 

 eighteenth hour, before the head process appears. 



In another series of experiments Assheton allowed the embryos 

 to remain in the incubator twenty-four to forty hours after a sable 

 hair had been inserted in the center of the unincubated blasto- 

 derm. In the embryos of twenty-four hours' incubation the hair 

 was found at the anterior end of the primitive streak, the medul- 



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