1 62 



FIRST FORMATION OF THE EMBRYO. 



process the primitive streak undergoes important changes both 

 in itself and in its relation to the 

 embryo. Its anterior thicker part, 

 which is enveloped in the diverging 

 medullary folds, soon becomes distin- 

 guished in structure from the part 

 behind this, and placed symmetrically 

 in relation to the axis of the embryo 

 (fig. 103, a.pr\ and at the same time 

 the medullary folds, which at first 

 simply diverge on each side of the 

 primitive streak, bend in again and 

 meet behind so as completely to enclose 

 the front part of the primitive streak. 

 The region of the embryo bird, where FIG. 103. DORSAL VIEW 



4.1 i 11 r 1J J- -1 OF THE HARDENED BLASTO- 



the medullary folds diverge, is known DERM OF A CHICK WITH FIVE 

 as the sinus rhomboidalis, though it MESOBLASTIC SOMITES. THE 



. . MEDULLARY FOLDS HAVE MET 



has no connection with the similarly FOR PART OF THEIR EXTENT, 

 named structure in the adult. By the BUT HAVE NOT UNITED - 

 time that ten somites are formed the a -P r - anterior part of the 



, 1-11. primitive streak; p.pr. pos- 



Sinus rhomboidalis IS completely CS- terior part of the primitive 



tablished, and the medullary groove streak - 

 has become converted into a tube till close up to the front end 

 of the sinus. In the following stages the closure of the 

 medullary canal extends to the sinus rhomboidalis, and the 

 folding off of the hind end of the embryo from the yolk 

 commences. Coincidently with the last-named changes the 

 sides of the front part of the primitive streak become thickened, 

 and give rise to conspicuous caudal swellings ; in which 

 the layers of the embryo are indistinguishably fused. The 

 apparently hinder part of the primitive streak becomes, as more 

 particularly explained in the sequel, folded downwards and 

 forwards on the ventral side. 



This is a convenient place to notice remarkable appearances which 

 present themselves close to the junction of the neural plate and the primitive 

 streak. These are temporary passages leading from the hinder end of the 

 neural tube into the alimentary canal. They vary somewhat in different 

 species of birds, and it appears that in the same species there may be 

 several openings of the kind, which appear one after the other and then 



