460 EMBRYOLOGY OF THE CHICK 



medullary plate. The cephalic ends of these plates seem to bend; but 

 from Hensen's node caudad, they diverge into thickenings on each side 

 of the primitive streak. 



At twenty-one to twenty-two hours the outer portions of the neural 

 plate bend dorsally toward the midline and form the neural or medul- 

 lary groove, the ridges thus formed being called the neural or medullary 

 folds (Fig. 266, B). This is the first differentiation of the nervous 

 system. 



After this period of incubation the denser portion which has formed 

 by all the cell differentiation mentioned above, is called the embryonal 

 area, and the outer peripheral region of the blastoderm is called the 

 extra-embryonic area, because from this extra-embryonic region arise 

 those structures which are not part and parcel of the embryo itself, but 

 serve as protective and nutritive layers. 



At this period the anlage of the head appears as a rounded eleva- 

 tion with a definite crescent-shaped head-fold, the first definite boundary 

 of the growing embryo. 



It is well at this point to 



know what is to become of trie 

 mesoderm, so that we may have 

 several landmarks which will 



chc' 



wr. I*. \_ ^^ -^*-- -^~^_ " r'Ti'ftTawimigy tun iuu'Jiift u' >'JL_LL_ E 1 



stand us in good stead. 



In the earthworm, it will be 

 recalled, the entire animal is seg- 



Sagittal section through region of primitive niented, that IS, COmpOSed of 

 node and caudal end of chordal canal of guinea mfz+imfrtto rVii1^ i'n tVi^ frr\rr coo- 



pig (isy, days after fertilization) to show be- metameres, while in me trog, seg- 



ginning of notochordal cells and ectodermal cells mentation SHOWS itself primarily 



in one layer. Let., ectoaerm ; ent., entoderm ; 



ch.c., chordal canal, dorsal and ventral wall clos- in the Spinal Column. 



ing lumen; pr.pt., primitive pit. (After Huber 



in The Anatomical Record, April 20, 1918.) In both earthworm and fl'Og 



the segments are composed of an outer layer of ectoderm, an inner layer 

 of entoderm, and a middle layer of mesoderm. 



When one speaks of metameres, one always means segments lying- 

 one behind the other, but now we must think of a sort of segmentation 

 also in each metamere, one below the other (Fig. 268). In fact, this we 

 must do if we are to understand that which follows. 



Figure 268 shows a combination transverse and longitudinal ar- 

 rangement of metameres with the mesoderm divided into an outer 

 (somatopleure) and an inner (splanchnopleure) layer, and the segments 

 also divided horizontally. 



The more dorsal portion of the horizontal divisions is called an 

 epimere, the mid-portion a mesomere (which is the beginning of the ex- 

 cretory system), and the more ventral portion is known as a hypomere. 

 The whole metamere is called a mesomeric somite. 



In vertebrates, as we have seen, segmentation is observable primarily 



