no THE BIOLOGY OF THE FROG chap. 



tary canal and its diverticula. The connective tissue, mus- 

 cular, and peritoneal layers are derived from the mesoblast. 

 For the most part it is composed of but a single layer of 

 cells. The epithelium of the mouth and a small portion of 

 the cloaca are produced by the ectoderm, these being the 

 only portions of the lining of the alimentary canal not of 

 entodermic origin. 



Organs from the Mesoderm. — The development of the 

 mesoderm has been traced to the stage in which it consists 

 of two double-layered sheets of tissue extending from the 

 notochord above to the ventral side of the body. The two 

 sheets of mesoderm are separated by the notochord except 

 for a short distance in front of and behind this structure, 

 where they become continuous across the middle line. A 

 division soon occurs in the mesoderm, separating a dorsal 

 portion, known as the vertebral plate, from a ventral part, 

 called the lateral plate. The former becomes divided trans- 

 versely into a number of blocks called myotomes, or muscle 

 segments. Each of these becomes thickened so that the cen- 

 tral cavity becomes reduced in size and finally disappears. 

 The division of the vertebral plate into segments begins in 

 the neck region of the embryo and proceeds backward. 

 The segments soon become separated from each other by 

 septa of connective tissue which assume the form of a V 

 with its apex pointing toward the anterior end of the body. 

 The myotomes are easily seen at the sides of the body of a 

 young tadpole, especially in the region of the tail. The cells 

 of the myotomes elongate in a direction parallel with the 

 long axis of the animal and become converted into muscle 

 fibers. 



The two layers of the lateral plates become widely sepa- 

 rated by the enlargement of the intervening body cavity 

 or ccelom. The inner or splanchnic layer becomes closely 



