466 



EMBRYOLOGY OF THE CHICK 



4. The neural groove almost but not quite closed. 



5. A clearly outlined fore-gut and mid-gut. 



6. Clearly defined head-folds marking the anterior limit of the embryo. 



7. A definite notochord extending from the anterior end of the primi- 



tive streak to what is to become the mid-brain. 



8. The pellucid area is more or less pear-shaped and the vascular area 



is seen as an inner zone of the area opaca. 



9. The primitive streak is rapidly becoming relatively shorter and is 



soon to disappear, the cells of which it is composed probably be- 

 coming rearranged to form other structures. 



Transverse section through the primitive streak of a chick with six pairs of 

 mesodermal somites (about twenty- four hours), showing the formation of the blood- 

 vessels and blood. The section extends from the mid-line, nearly half across the 

 area vasculosa. b, Blood cells ; c, coelomic spaces ; e, empty endothelial tubes ; ec. 

 ectoderm ; en, endoderm ; gw, germ wall ; i, solid blood island ; m, axial mesoderm ; 

 i , . a, primitive streak ; si, vascular sinuses of area vasculosa ; so, somatic mesoderm ; 

 sp, splanchnic mesoderm. (After Riickert.) 



From the twenty-fourth hour on, the texture of the embryo becomes 

 firmer, and, whereas, it is difficult to remove an eighteen-hour embryo 

 without tearing, the twenty-four hour embryo can easily be removed. 

 All outlines become clearer now also. 



The anterior part of the embryonal area has thickened, and is 

 slightly lifted above the remaining blastoderm as shown by the crescent- 

 shaped anterior boundary (Fig. 266, B). The embryo grows forward 

 oyer this crescent-shaped fold which thus comes to lie under the embryo 

 and forms a little pocket between the embryo and the fold called the 

 subcephalic pocket. 



The neural folds now unite in the region of the future mid-brain, 

 closing rapidly posteriorly, and slowly anteriorly. The closed portion 

 is called the neural tube. The most anterior portion where the neural 

 tube will close is known as the neuropore, which is the region of what 

 is later to become the lamina terminalis. This lamina terminalis is 

 usually regarded as the morphologically anterior limit of the brain. 

 Topographically, however, this is not the case, for the fore-brain grows 

 forward and then bends back downward in front of the fore-gut, the 

 whole thing becoming bent like a shepherd's crook, so that the morpho- 

 logically anterior end comes to lie on its antero-ventral aspect. 



The neural folds have a somewhat flattened crest, and these fold in- 

 ward, forming a vertical contact. The neural tube is thus formed by the 



