The Development of the Vertebrate Embryo 65 



DEVELOPMENT OF THE DIGESTIVE AND RESPIRATORY SYSTEM 



At first the gut is a simple tube stretching from mouth to anus. At 

 the anterior end, five folds appear on each side which penetrate and 

 finally pierce the mesoderm and make contact and fuse with the outer 

 ectoderm. Slits appear where endoderm and ectoderm meet. These are 

 the gill slits. In fish they remain as functional organs, but in higher verte- 

 brates they disappear and the component tissues are transformed into 

 other organs. The lungs also originate from the anterior gut, first as a 

 simple outpocketing which soon branches into two bag-like structures. 



Similar pocketings, extensions, and local swellings in the posterior 

 portion of the gut result in the formation of stomach, intestines, liver, etc. 



DEVELOPMENT OF MESODERMAL STRUCTURES 



By the end of gastrulation, the mesoderm makes up the bulk of em- 

 bryonic tissue. The notochord is the first mesodermal organ to appear, 

 and the remainder of the mesoderm is spread out as a thin sheet lying 

 between the ectoderm and endoderm. This sheet breaks up into longitu- 

 dinal strips. The thick strips lying on both sides of the notochord become 



Fig. 34. Advanced em- 

 bryos: (A) the chick, (B) 

 the salamander, and (C) 

 the human. 



