The Development of the Frog 69 



where they approach more nearly the condition 

 in man and other mammals. 



The spleen, since it is so intimately associated 

 with the blood, may be mentioned at this time. 

 " The spleen arises as a spherical bud on the 

 mesenteric artery : it consists of cells similar to 

 those of the lymphatic tissue, and, like these, 

 is said to be derived originally from the ento- 

 blast cells of the mesenteron." 1 



DEVELOPMENT OF THE CCELOM AND THE MUS- 

 CULAR SYSTEM 



The formation of the mesoblast as a layer 

 of cells between the ectoblast and entoblast, 

 and the splitting of this mesoblast into two 

 layers, the somatopleure and splanchnopleure, 

 with the body-cavity or ccelom between them, 

 has already been mentioned (Fig. 15). 



The sheet of mesoblast on each side of the 

 body rapidly grows ventralward until it meets 

 and fuses with its fellow of the opposite side, 

 so that there is very early a complete layer 

 of mesoblast over the ventral side of the 

 embryo (Figs. 13, M, and 15). Along the 

 mid-dorsal line, however, the two sheets of 

 mesoblast remain distinct, being separated by 



1 Marshall. 



