V THE DEVELOPMENT OF THE FROG iii 



applied to the entoderm of the archenteron and forms the 

 supporting tissue and musculature of the alimentary canal 

 and its diverticula. The outer or somatic layer comes to lie 

 against the outer ectoderm and forms the inner portion 

 (connective tissue, muscle, and peritoneum) of the body wall. 

 The innermost portion of both the somatic and splanchnic 

 layers of mesoblast become differentiated as a separate 

 layer, the peritoneum, which is continuous all around the body 

 cavity. As the right and left halves of the coelom arise inde- 

 pendently and gradually extend toward the mid-ventral line, 

 they are separated for a time by a median ventral partition. 

 This subsequently breaks down along most of the length of the 

 alimentary canal, putting the two sides of the body cavity in 

 connection with each other. The median partition persists, 

 however, for a short distance anteriorly, forming the vertical 

 membrane which extends from the liver and pericardium 

 to the ventral body wall. A still smaller portion occurs 

 between the body wall and the ventral side of the cloaca. 



The heart and pericardium take their origin from the 

 mesoderm near the anterior end of the ventral side of the 

 body. A pair of fissures appears in the sheet of mesoderm 

 in this region ; these gradually enlarge and extend toward 

 the middle line. The layer roofing over these fissures be- 

 comes raised up on either side, and the two folds thus formed 

 meet each other above, forming a sort of tube. Within this 

 tube are inclosed some scattered cells which arrange them- 

 selves into a layer that becomes the endothelial lining of the 

 heart. The cavity outside the tube becomes the cavity of 

 the pericardium, and the tube itself thickens and becomes 

 transformed mainly into the heart, but its outer layer gives 

 rise to a thin sheet of tissue, the visceral portion of the peri- 

 cardium. The tissue which at first connects the heart with 

 the ventral side of the pericardium becomes broken through. 



