THE ONTOGENESIS OF VERTEBRATES 65 



connective tissue, and form the vascular system (blood-vessels 

 and lymphatics). 



The arrangement of the various embryonic elements at this 

 point is shown in the accompanying diagrams based upon 

 selachian embryos, and exhibiting the actual proportions as 

 they exist in a rather primitive vertebrate. [Fig. 14.] The 

 general arrangement of parts in an adult dog-fish is not ma- 

 terially different from the last of these. Through the forma- 

 tion of a restricted middle area, the mesodermic diverticula 

 become divided into dorsal, middle and ventral portions, the 

 epimere, mesomere and hypomere respectively, each with a 

 distinct, separate history. 



The epimere, the inner wall of which becomes greatly thick- 

 ened, eventually cuts itself off from the remaining meso-hypo- 

 mere, and expands both dorsally and ventrally between the 

 latter and the ectoderm until it meets the opposite one in the 

 mid-dorsal and mid-ventral lines, separated only by thin strips 

 of connective tissue. From the thickened inner wall of this 

 develop the voluntary muscles of the body, the segmentation 

 of which is retained among the fishes throughout the greater 

 part of the body, and still appears in unmistakable traces 

 among the highest forms. The mid-ventral connective tissue 

 partition separating the muscle masses of the two sides be- 

 comes the linea alba, a conspicuous white line, which persists 

 in all vertebrates. The cavity of the epimere becomes sup- 

 pressed by the growth of the inner wall and thus comes to 

 nothing. 



The consecutive meso-hypomeres soon lose their independ- 

 ence through the breaking down of the transverse partitions, 

 as described above, but the metameric repetition found among 

 the parts derived from them continues to suggest their origin 

 as separate diverticula. From the narrowed mesomere there 

 arise the essential organs of the urogenital system, many parts 

 of which retain throughout life the indications of a segmental 

 origin. The cavities of the mesomere become those of the 

 systems derived from it. 



The hypomeres, fused into a single bag or sac, form the 



