260 Basic Structure of Vertebrates 



thicker region becomes sharply folded upward. On either side of this 

 median fold, a longitudinal groove appears on the inner surface of the 

 endoderm. Then the endoderm in the region of each of these grooves 

 assumes the form of a fold extending outward dorsolaterally. Thus 

 arise three folds, one median and a lateral pair, all convex outward, 

 and extending nearly the whole length of the embryo. As time goes on, 

 these folds become more emphasized, but soon a difference arises be- 

 tween the median fold and the lateral folds. The median fold remains 

 continuous throughout its entire length. The lateral folds, however, 

 become interrupted by the formation of sharp, deep transverse folds 

 which cut from above downward through each lateral fold. This process 

 of subdivision or segmentation begins near the anterior ends of the 

 lateral folds. Its immediate result is a pair of approximately globular 

 pouches lying symmetrically on either side of the median fold, each 

 pouch having a small central cavity opening by a narrow passage into 

 the archenteron. Later this passage is closed and then the pouch be- 

 comes detached (Fig. 213F) from the archenteric wall which, at the 

 place where the pouch had formed, closes so that nothing is left to 

 mark the spot. 



Immediately behind each pouch of the first pair, another similar 

 pouch forms exactly as the first did. At this stage of development, 

 marked by the presence of two pairs of these pouches, the embryo 

 escapes from the egg membrane ("hatches"). The period between 

 fertilization and hatching varies considerably, its average being prob- 

 ably not far from 12 hours. 



These two pairs of pouches derived from the dorsolateral endo- 

 dermal wall of the archenteron constitute the first definitely delimited 

 mesodermal material. The remainder of the dorsolateral folds, extend- 

 ing back to the blastoporal region, is destined to give rise, after hatch- 

 ing, to additional mesodermal pouches. The median endodermal fold, 

 which has remained intact during this process of segmentation of the 

 lateral folds, is the material of the future notochord (Fig. 213, NC). 



At the time of hatching, then, the embryo has made important 

 progress beyond the gastrula stage. Not only has the segregation of 

 mesoderm begun but two important organs, the central nerve tube 

 and the notochord, are indicated. 



After hatching, additional pairs of mesodermal segments are cut off 

 from the lateral mesodermal folds, the addition taking place progres- 

 sively from anterior to posterior, until a total of ordinarily 14 pairs 

 have been produced. In several of the more posterior segments, cavi- 

 ties do not occur, the mesodermal folds merely breaking up into a suc- 

 cession of solid blocks of cells (Fig. 213G). 



In the formation of these 14 pairs of mesodermal pouches, the 



