germ-layers found in all 



the heart and the vascular system or blood-vessels, and the stom- 

 ach and intestines. The third and innermost layer is called 

 the " hypoblast," By the sixth or eighth hour these three mem- 

 branes become definitely established. The middle layer now 

 thickens and thus causes the appearance known as the " primitive 

 streak," along the middle of which runs the depression known as 

 the " primitive groove." In front of the primitive groove appears 

 the " medullary groove," and below it the notoehord or " chorda 

 dorsalis" originates from the cells of the middle layer. This 

 notoehord (Fig. 304, ch) lies directly beneath the medullary tube 



(mr) and between the outer and third germ-layer in the form of a 

 flattened circular rod. The blastoderm is now folded anteriorly 

 like the letter S ; this is called the " head-fold," and soon after 

 the "tail-fold " is formed in a similar way. These two folds meet 

 in the middle thus forming the body of the embryo. 



Next the primitive groove and streak disappear as the sides of 

 the medullary groove rise up, when they finally meet, forming the 



About this period the first pair of protovertebrse make their 

 appearance. They arise from the mcsoblast as two cubical masses 

 (Fig. 304 1 , u iv) lying one on each side of the -notoehord. Two 

 more pairs appeal- hrhind the fust pair before the first day is 



