l62 



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 

 the medullary folds diverge, is known 

 as the sinus rhomboidalis, though it 

 has no connection with the similarly 

 named structure in the adult. By the 

 time that ten somites are formed the 

 sinus rhomboidalis is completely es- 

 tablished, and the medullary groove 

 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 



FIG. 103. DORSAL VIEW 

 OF THE HARDENED BLASTO- 

 DERM OF A CHICK WITH FIVE 

 MESOBLASTIC SOMITES. THE 

 MEDULLARY FOLDS HAVE MET 

 FOR PART OF THEIR EXTENT, 

 BUT HAVE NOT UNITED. 



a.pr. anterior part of the 

 primitive streak ; p-pf- pos- 

 terior part of the primitive 

 streak. 



