832 



DEVELOPMENT 



[CH. LIX. 



surface. In the meantime, a second groove has appeared on the 

 surface of the ovum in front of the primitive streak. This is the 



FIG. 624. Diagram of a surface 

 view of a young mammalian 

 blastula. 1, Germinal area. 

 A, line of section represented 

 in fig. 625. 



Fia. 625. Diagram of a 

 section of the mammalian 

 blastula shown in fig. 624 

 along the line A. 1, Ger- 

 minal an .1 ; 2, epiblast ; 

 3, inner cell mass. 



neural groove or rudiment of the central canal of the brain and spinal 



cord. It is bounded by two folds 

 the neural folds, which are united 

 together at their anterior ends, but 

 their posterior ends which embrace 

 the anterior end of the primitive 

 streak do not unite until after the 

 appearance of the opening at the 

 anterior end of the primitive streak. 

 This opening therefore connects the 

 neural groove with the cavity in the 

 interior of the blastodermic vesicle, 

 which is called the archenteric cavity, 

 and through it the epiblast be- 

 comes continuous with the hypoblast. 

 Therefore it evidently represents a 

 part of the blastopore of more 



primitive forms, but it is called the neurenteric canal. It soon 



closes, and all traces of it are lost. 



FIG. 626. Diagram of a surface view of a 

 mammalian blastoderm after the formation 

 of the neural groove. 1, Germinal area ; 2, 

 neural ridge ; 3, neural groove ; 4, neuren- 

 teric canal (part of blastopore) ; 5, primitive 

 groove and streak. A, line of section shown 

 in fig. 627; B, line of section shown in 

 fig 628. 



FIG. 627. Diagram of a transverse section 

 through a mammalian blastoderm along 

 line A in fig. 626. 1, primitive groove ; 2, 

 primitive streak ; 3, epiblast ; 4, mesoblast ; 

 5, hypoblast ; 6, coelom ; 7, archenterou. 



7 B 



FIG. 623. Diagram of a transverse section 

 through a mammalian blastoderm along 

 line B in fig. 626. 1, neural groove ; 2, 

 neural ridge ; 3, epiblast ; 4, somatic 

 mesoblast ; 5, splanchnic mesoblast ; 6, 

 hypoblast ; 7, somatopleur : 8, splanch- 

 uopleur ; 9, notochord ; 10, coelom ; 11, 

 archenteron. 



The primitive streak itself is due to a down-growth of a linear 



