into the Growth of the Blastoderm of the Chid'. 353 



margin of the area pellucida. The area opaca takes no part at all in 

 the formation thereof. 



I may add that from a careful examination of surface views of 

 living and preserved specimens, and from sections, I find it just as 

 difficult to corroborate Duval's account of the formation of the 

 primitive streak as I do from the experimental study I have just 

 described. 



I come now to the second part of the inquiry ; namely, what part 

 of the actual embryo does the primitive streak give rise to ? 



A sable hair inserted at the centre of the blastoderm appears at the 

 anterior end of the primitive streak. 



If such a specimen is allowed to develop for some hours longer, 

 until the medullary plate and medullary groove are clearly formed, 

 these structures are found to be in front of the sable hair ; that is to 

 say, the sable hair is still at the front end of the primitive streak 

 (fig. B). If a specimen, in which the sable hair has been inserted at 

 the same spot that is to say, at the centre of the unincubated 

 blastoderm is left until several pairs of mesoblastic somites have 

 appeared, the hair is found at the level of the most anterior pair of 

 somites (fig. B (iii)). 



From these specimens it seems clear that all those parts in front of 

 the first pair of mesoblastic somites (that is to say, the heart, the 

 brain, and medulla oblongata, the olfactory, optic and auditory 



FIG. B. (i) Diagram of unincubated Blastoderm, 

 (ii) Blastoderm after 24 hours' incubation, 

 (iii) Blastoderm after 40 hours' incubation. 



AO, area opaca j AV, area vasculosa ; N"Gr, medullary groove ; PS, primitive streak 

 X, point of insertion of sable hair. 



