ORIGIN OF THE MESODERM 



461 



?n the region of the spinal column. Therefore, in the study of verte- 

 brates, such as the chick, we shall find that, while segmentation begins 

 along the future spinal region, only the more dorsal portion of the meso- 

 derm is segmented, and that only partially. The epimeres alone, that 

 is, the paired parts lying at the side of the notochord, are truly seg- 

 mented, though the opening in them, the epicoele, shortly disappears. 

 The mesomeres with their mesocoeles develop into the excretory sys- 

 tem, and the hypomeres, which have not segmented, but whose opening, 

 the hypocoele, is continuous throughout the entire region where there 

 has been any segmentation, is now to be known as the coelom or body 

 cavity, into which the internal organs are to grow. 



Fig. 268. 



Stereogram showing the segmentation of the mesothelium. The dorsal and 

 ventral walls of the coelom later fuse to form the dorsal and ventral mesenteries. 

 AJ, alimentary canal ; EM, epimere ; Fb, forebrain ; Hb, hindbrain ; M, (under 

 SA-.c. ), myotome ; Mb, midbrain ; MM, mesomere ; sk.c., sclerotome. Sp, splanchnic 

 layer of the mesoderm (splanchnopleure). (Modified from JCingsley. ) 



It is to be remembered that epimere, mesomere, and hypomere are 

 composed of mesoderm only. 



As the mesoderm begins to grow laterad and ventrad, and while it 

 is yet unsplit into an outer and inner layer, the thickened portion lying 

 on each side of the neural groove is called the vertebral plate, and the 

 more distal portion, the lateral plate. 



The outer layer of the lateral plate, after it splits into two sheets, 

 is called somatic mesoderm (and after connecting with the ectoderm the 

 somatopleure) while the inner layer, the splanchnic mesoderm, connects 

 with the entoderm and is known as the splanchnopleure (Fig. 268). 



In the head region, the cells of the vertebral plate scatter and com- 

 bine with cells which are continually being budded off from the walls 

 of the fore-gut to form the mesenchyme of the head region (Fig. 269). 

 It will thus be seen that mesenchyme is made up of a combination of 



