THE DEVELOPMENT OF THE INDIVIDUAL 



529 



Variations in the Method of Mesoderm Formation 



The presence of large amounts of yolk modifies the method of 

 mesoderm formation as well as the method of cleavage. The method 

 in the embryonic chicken has been extensively studied as an example 

 of this modified form of mesoderm formation. A mass of cells forms 

 between the endoderm and ectoderm from the primitive streak and 

 spreads out laterally. This is the mesoderm. Later a space develops on 

 each side by a splitting of this cell mass into two parts. These spaces 

 later fuse and produce the coelom. The coeloms produced in this way 

 are identical in appearance with those produced in the manner described 

 for Amphioxus. 



Aorta 



Somite Kidney 



D 



Fig. 33.5. Variations in method of coelom formation. (A) Starfish and (B) Am- 

 phioxus represent the enterocoelic method. (C) Amphibian and (D) bird and mam- 

 mal represent the schizocoelic method. The coelom is shown in black. 



The further development of the mesoderm is very important, because 

 this layer forms by far the largest number of organs in the body. The 

 mesoderm gradually separates into three regions, each of which de- 

 velops many important parts of the growing animal. These are : the 

 dorsal mesoderm, the intermediate mesoderm, and the ventral mesoderm. 

 The dorsal mesoderm forms the somites, which function in muscle and 

 skeleton formation. The intermediate layer gives rise to the kidneys. 

 The ventral portion splits into the splanchnic mesoderm which fuses to 

 the digestive tract and the somatic mesoderm which lines the body 



