354 Mr. R. Assheton. An Experimental Examination 



organs and fore gut) are developed from that portion of the unincu- 

 bated blastoderm which lies anterior to the centre of the blastoderm, 

 and that all the rest of the embryo is formed by the activity of the 

 primitive streak area. 



I have found it very difficult to determine, exactly, the anterior 

 limits of the embryo in the unincubated blastoderm. This, no doubt, 

 is due to the fact that, for the production of the anterior end of the 

 embryo, very complicated foldings of the blastoderm are called into 

 play, and the insertion of a bristle or the infliction of any injury to 

 the delicate parts of the blastoderm involved in the process, almost 

 entirely prevents anything like a normal course of development. 



However, such little success as I have had gives the following 

 results : A hair inserted at the most anterior border of the area 

 pellucida is found far in front of the primitive streak. 



A hair inserted only slightly in front of the centre of the blasto- 

 derm appears (in a specimen in which the medullary folds are just 

 becoming visible) in the medullary plate in front of the primitive 

 streak. In older specimens, after the head -fold has been formed, 

 the embryos are extremely abnormal when the sable has been inserted 

 in the region under discussion. 



Indeed, very few will develop as far as the formation of the head- 

 fold. 



The only facts I can derive from the insertion of sable hairs in 

 this area are : 



( 1) That it interferes very seriously with the course of develop- 



ment. 



(2) That the bristle appears inside the two anterior horns of the 



area vasculosa. 



(3) That if placed some little way anterior to the centre it is found 



apparently in front of the embryo, but it interferes so greatly 

 with the head-fold that it is difficult to say whether it has, 

 or has not, perforated the anterior part of the embryo. 



I have shown that a hair inserted between the centre of the 

 blastoderm and the hinder margin of the area pellucida is found after 

 about twenty hours of incubation in the primitive streak. When a 

 specimen in which the sable has been similarly placed is allowed 

 to develop until several mesoblastic somites have been formed, it is 

 found to be posterior to the first formed mesoblastic somites. 



For instance, in the specimen with me, the blastoderm measured 

 4*3 mm. in diameter. The sable was inserted 1'3 mm. from the 

 posterior edge of the blastoderm. After forty-one hours of incuba- 

 tion seven pairs of mesoblastic somites had been formed, and the 

 sable hair was a short distance posterior to the 7th pair of somites. 



From such specimens as these we are, I think, bound to conclude 



