GENERAL FEATURES OF CHORDATE DEVELOPMENT 39 



the archenteron, a mass of cells will be seen protruding dorsally; or in some 

 slides this mass of cells may have separated from the archenteron and lies between 

 the latter and the neural tube. This mass is the notochord. Between the ecto- 

 derm and the archenteron on each side is a narrow sheet of cells extending irom 

 the sides of the neural tube ventrally. In some slides these sheets will extend 

 only a short distance, while in others they reach nearly to the median ventral 

 line. These sheets are the mesoderm. Make a diagram of the section, coloring 

 ectoderm blue, entoderm yellow, and mesoderm red. 



E. FURTHER HISTORY OF THE MESODERM 



The history of the mesoderm is of the utmost importance for the understand- 

 ing of vertebrate structure. We have already noted that the mesoderm splits 

 into two layers, an outer or somatic layer, and an inner or splanchnic layer, and 

 that the space between the two layers is the body cavity or coelom. The meso- 

 derm grows from each side of the embryonic axis ventrally to the median ventral 

 line, or in meroblastic eggs grows out over the yolk, pushing out between ecto- 

 derm and entoderm. 



The mesoderm next becomes differentiated into three regions: a dorsal region, 

 called the epimere, which lies to each side of the neural tube; a middle region, 

 called the mesomere or nephrotome, situated lateral and ventral to the epimere; 

 and a large ventral region on each side of the archenteron, called the hypomere 

 or lateral plate. Each of these regions has of course both somatic and splanch- 

 nic walls (see Fig. &4, also K, p. 14). The epimere immediately becomes seg- 

 mented, that is to say, dorsoventral clefts appear in it at regular intervals, the 

 process beginning at the anterior end of the embryo and proceeding posteriorly. 

 Consequently, the epimere becomes divided up into a longitudinal row of blocks, 

 a row on each side of the neural tube. These blocks are epimeres, or mesoblastic 

 somites (originally called provertebrae as it was erroneously supposed that they 

 were primitive vertebrae). At first the epimeres are still continuous ventrally 

 and laterally with the mesomere, but eventually they are completely cut off 

 from the rest of the mesoderm (see Fig. SB and the figure in K, p. 15). The 

 mesomere and the hypomere do not become segmented and remain permanently 

 in close relation to each other. Within the mesomere little tubules appear, which 

 open into the cavity of the hypomere; they are the tubules of the kidney (see 

 Fig. SB). The hypomeres of each side fold around the archenteron, their inner 

 walls coming in contact above and below the archenteron to form double-walled 

 membranes, the dorsal and ventral mesenteries (see Fig. SB). The cavities of 

 the two hypomeres become the coelom of the adult; the cavity in each epimere 

 disappears; and that of the mesomere remains as the cavities of the tubules of 

 the kidney. 



In embryos of the amphibian type the archenteron is a closed tube and the 

 two hypomeres are closed cavities which meet below the archenteron. In 



