STRUCTURE OF THE VERTEBRATES 67 



The mesoderm follows immediately after the beginning of the 

 notochord. It arises as two groups of somites on either side of the 

 notochord, and grows lateralh'. As the two sheets of mesoderm 

 spread between the ectoderm and endoderm the outer portion 

 splits (recall the enterocoel of Amphioxus, and the cavity in the 

 frog mesoderm). As growth proceeds the upper layer of meso- 

 derm follows the ectoderm, and the lower adheres to the en- 

 doderm. 



The embryo is now taking shape, and as differentiation of 

 organs proceeds the peripheral cells divide, carrying embryonic 

 tissues around the yolk. Eventually these membranes of em- 

 bryonic origin completely surround the yolk mass, the inner 

 membrane composed of endoderm and one layer of mesoderm 

 being the yolk sac. The outer membrane is mesoderm and ecto- 

 derm. However, although these membranes are of embryonic 

 origin, only the original region forms the embryo, the outlying 

 tissues being extra-embryonic and are lost at hatching time. 



The embryo grows more rapidly at the anterior end, and as 

 the neural groove closes and enlarges to form the brain it lifts 

 up from the surrounding tissues. The posterior end then lifts off 

 and the sides of the embryo bend downward and constrict ven- 

 trally. These processes lift the embryo from the extra-embryonic 

 membranes, so that the embryo is raised above the still spherical 

 yolk. Recall that the ectoderm, mesoderm and endoderm of the 

 embryo are continuous with the membranes which spread around 

 the yolk mass, and the archenteron is still open ventrally above 

 the yolk sac. Anterior and posterior growth accomplishes two 

 results: (1) the two ends of the archenteron are pulled out so 

 that a distinct gut is formed; and (2) the original point of con- 

 nection between the embryo and yolk is left as a circular body 

 stalk. The connection between the gut and the yolk sac continues 

 until the time of hatching, but as the body stalk increases only 

 slightly in diameter and the embryo grows rapidly, the con- 

 nection is relatively very slight after the first few days. 



Amnion and Allantois. These two distinctive structures of 

 the amniotes (reptiles, birds and mammals) have begun their de- 

 velopment at this time. Recall that the extra-embryonic tissues, 

 continuous with those of the embrvo's bodv, form two sheaths 



