492 DEVELOPMENT OF PRIMITIVE BODY FORM 



area of the anterior trunk and posterior hindbrain region of the embryo. In 

 the chick embryo (see Patterson, '07), the most anterior segment forms first, 

 and later segmentation progresses in a caudal direction. This probably holds 

 true for most other vertebrates. However, in elasmobranch fishes, segmenta- 

 tion of the epimeric mesoderm also extends forward from the hindbrain 

 area into the head region presenting a continuous series of somites from the 

 eye region caudally into the tail (fig. 217D). (Study figs. 217D, 230D.) Seg- 

 mentation of epimeric mesoderm appears in the head region of Amphibia. 

 In many higher vertebrates, three pairs of somitic condensations appear in 

 the area just caudal to the eye but at a slightly later period of development 

 than that of the elasmobranch fishes (fig. 217D-F). 



2) Mesomere. The narrow longitudinal band of mesoderm, adjoining the 

 lateral border of the epimere, is the mesomere (figs. 22 IF, G; 230D; 234E, F). 

 This mesoderm ultimately gives origin to much of the excretory (kidney) 

 tissue and ducts and to certain of the reproductive ducts of many vertebrates. 

 (See Chap. 18.) Because of the origin of nephric tissue from its substance, 

 this longitudinal band of mesoderm generally is referred to as the urogenital 

 or nephrotomic mesoderm. Synonymous terms often used are intermediate 

 mesoderm or intermediate cell mass. The mesomere undergoes a segmentation 

 similar to the epimeric area in its more anterior portion where the pronephric 

 kidney develops in higher vertebrates, while in lower vertebrates, such as the 

 shark embryo, it may be more extensively segmented. 



3) Hypomere. The remainder of the mesoderm which extends latero- 

 ventrally from the mesomere forms the hypomere or hypomeric mesoderm. 

 It also is called the lateral plate mesoderm or lateral plate mesoblast. This 

 portion of the mesoderm does not become segmented in present-day verte- 

 brates. (Compare with the condition in Amphioxus described on p. 505.) 



b. Tabulation of the Mesodermal Areas 



Coincident with the formation of the somites, a cavity begins to appear 

 within the mesoderm. This cavity or primitive coelomic space separates the 

 mesoderm into two layers, an outer layer near the ectoderm and an inner layer 

 close to the neural, notochordal, and entodermal cells. This hollowing process 

 within the mesodermal layer is known as coelom formation or tubulation 

 of the mesoderm. In many embryos of the lower vertebrates, there is a strong 

 tendency for the coelomic space to form throughout the entire lateral mass 

 of mesoderm from the epimeric area ventrad into the lateral plate mesoderm. 

 For example, in elasmobranch (shark) embryos of about 3 to 4 mm. in length 

 and also in many early post-gastrular amphibia, the following features of the 

 primitive coelom are found in the trunk region of each mesodermal mass: 



( 1 ) The mesoderm possesses a cavity, continuous dorso-ventrally from 

 the epimere into the lateral plate (figs. 217G, H; 22 IE). When the 

 epimere (and to some extent the nephrotomic region as well) under- 



