CHANGES DURING FIEST DAY. ' 415 



mesoblast originates as two masses from the hypoblast, and the 

 notochord referred to below arises in a similar way and at the 

 same time. Before the end of the twelfth hour a streak appears 

 at one end of the area pellucida, the primitive streak, which is 

 due to the thickening of the middle portion of the blastoderm by 

 rapid cell-division of the epiblast. About this time the germinal 

 area alters in appearance, the area pellucida becoming oval, and 

 then about the fifteenth hour pear-shaped, and the primitive 

 streak becomes the primitive groove by a lateral fold arising 

 on each side. Between the seventeenth and twentieth hours 

 there appears an axial opaque line below the primitive streak ; 

 this is the notochord (Not), which is composed of a number of 

 concentrated cells. There then appears a groove in front of the 

 primitive streak, the medullary groove (md), in a thickened epi- 

 blastic plate, which gives rise to the central nervous system ; 

 this is plainly seen about the twentieth hour. At the same time 

 there will be noticed a fold in front of the head-fold that is com- 

 mencing to appear ; this is the amnion. From now to the end 

 of the first day development proceeds rapidly. The head is 

 definitely commencing to form. In front the medullary folds 

 unite in the region of the future brain, forming a canal, the 

 neural canal. The mesoblast on each side of the notochord 

 beneath the medullary folds becomes cut up into a number of 

 cubical plates ; these are called mesoblastic somites, from which 

 the voluntary muscles of the trunk and vertebrae are formed. 

 The embryo now grows rapidly, and the primitive streak grows 

 backwards. Similarly changes have taken place in the area 

 opaca during the first twenty-four hours. The area opaca has 

 spread out over the yolk and reaches the size of a sixpence, and 

 that portion of it nearest the area pellucida can be told by its 

 mottled appearance ; this wiU become the vascular area of the 

 embryo. 



Changes during the second day. — During the second day the 

 embryo presents very marked changes. The head end becomes 

 prominent. The medullary folds in the cephalic region are 



