III.] 



THE NOTOCHORD. 

 Fia. ii. 



SURFACE VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18 HOURS. 



None of the opaque area is shewn, the pear-shaped outline indicating the 

 limits of the pellucid area. 



At the hinder part of the area is seen the primitive groove pr., with its 

 nearly parallel walls, fading away behind, but curving round and meeting in 

 front so as to form a distinct anterior termination to the groove, about half way 

 up the pellucid area. 



Above the primitive groove is seen the medullary groove m. c., with the 

 medullary folds A. These diverging behind, slope away on either side of the 

 primitive groove, while in front they curve round and meet each other close 

 upon a curved line which represents the head-fold. 



The second curved line in front of and concentric with the first is the com- 

 mencing fold of the amnion. 



groove, however, the mesoblast is thinned out and very soon 

 the cells in this position, separating from the lateral masses, 

 adhere together in the middle line, and thus form between 

 the epiblast and the hypoblast a flattened circular rod known 

 as the notochord, seen in section as an elliptical aggregation 

 of cells (Fig. 12, cA.) 



E. 4 



