STRUCTURE OF TWENTY-FOUR HOUR CHICKS 47 



tionships of fore-gut to the head. From the sagittal section it 

 will also be apparent why the margins of the intestinal portal 

 and of the subcephalic pocket appear as dark lines in the whole- 

 mount. In viewing an entire embryo under the microscope by 

 transmitted light one depends largely on differences in density 

 for locating deep-lying structures. When a layer is folded so 

 the light must pass through it edgewise, the fold stands out as a 

 dark line by reason of the greater thickness it presents. 



The Regional Divisions of the Mesoderm. The first con- 

 spicuous metamerically arranged structures to appear in the 

 chick are the mesodermic somites. The somites arise by divi- 

 sion of the mesoderm of the dorsal or segmental zone to form 

 block-like cell masses. In the embryo shown in Figure 15 three 

 pairs of somites are completely delimited and a fourth pair can 

 be made out which is not as yet completely cut off from the 

 dorsal mesoderm posterior to it. 



The regular addition of somites as embryos increase in age 

 makes the number of somites the most reliable criterion of the 

 stage of development. Chicks which have been incubated for 

 a given number of hours show wide variation in the degree of 

 development attained; chicks of a given number of somites 

 vary but little among themselves. ' 



Cross sections passing through the mid-body region show the 

 formation of the somites and the beginning of other changes in 

 the mesoderm (Fig. 17, C, cf. also Fig. 28, E). Following the 

 mesoderm from the mid-line toward either side three regions or 

 zones can be made out: (i) the dorsal mesoderm which at this 

 level has been organized into somites, (2) the intermediate 

 mesoderm, a thin plate of cells connecting the dorsal and lateral 

 mesoderm and (3) the lateral mesoderm which is distinguished 

 from the intermediate by being split into two layers with a space 

 between them. 



The somites are compact cell masses lying immediately 

 lateral to the neural folds. The cells composing them have a 

 fairly definite radial arrangement about a central cavity which 

 is very minute or wanting altogether when the somites are first 

 formed but which later becomes enlarged (Fig. 38). Cephalic 

 and caudal to the region in which somites have been formed the 

 dorsal mesoderm is differentiated from the rest of the mesoderm 

 simply by its greater thickness and compactness. 



