ELISHA MITCHELIv SCIENTIFIC SOCIETY. 69 



PRIMITIVE STREAK AXD BLASTOPORE OF 

 THE BIRD EMBRYO. 



BY H. V. WILSON. 



Embryologistrj, with but few exceptions, recognize 

 in the bird embryo a g-astrula stag-e. There is, how- 

 ever, a very considerable diversity of opinion as to just 

 what constitutes the g-astrula. Leaving aside certain 

 interpretations for which at present there seems no 

 g-ood ground, we find there are two ver}^ different views 

 held regarding the nature of this embryonic stage. 



According to the older view, advanced b}^ Balfour 

 and Rauber, the essential difference between the bird 

 gastrula and the fish gastrula is that a part of the 

 original edge of the blastoderm, is in the bird turned 

 in to form the primitive streak. Thus while in the fish 

 the blastopore is represented b}^ the blastoderm edge, 

 in the bird it is represented by the primitive streak plus 

 the blastoderm edge. This theoretical view receives 

 the support of the well known researches of Duval on 

 the germ layers of birds\ Duval finds that the verv 

 3^oung blastoderm of the bird is similar to that of fishes. 

 In both, the ectoderm and entoderm are continuous 

 round the edge, which therefore corresponds to the 

 blastopore. But this precise similarit}^ is onlv tran- 

 sient, for in the bird the primitive streak soon makes 

 its appearance. The manner in which the primitive 

 streak is formed proves conclusively that it is onlv a 

 modified part of the blastoderm edge. The voung 

 blastoderm (fish-like stao-e) grows centrifusfallv at all 



1, De la formation du blastodertne dans I'oeuf d'oiseau. Annales 

 des Sciences Nat. Zoolog-ie. T. XVni.. 1884. 

 5 



