216 GENERAL ZOOLOGY 



the primary germ-layers, has very great significance. Already 

 we can note many changes from the simple blastula condition 

 to the advanced gastrula. In discussing these and subsequent 

 changes, we shall use the word " differentiation," which means 

 literally " a becoming different." Differentiation increases 

 after the gastrula stage, and the embryo becomes longer and 

 larger, as organ after organ makes its appearance out of one 

 or the other of the three germ-layers. However small the 

 organs may be, the trained embryologist knows with a high 

 degree of certainty the origin and history of each one of them. 



Before the gastrula has passed into a later stage we can 

 observe in sections cut like those in Figs. 105, C\ 1), E, F, that 

 the two large cells at the closed end of the gastrula, called 

 pole-cells, are giving rise by division to a row of mesodermal 

 cells. In the older portion of the two rows of mesodermal cells 

 cavities are beginning to appear (Fig. 106, A, I>). Fig. 106, ]> 

 shows how a gastrula of the same age appears in cross sec- 

 tion. Ectoderm and endoderm are separated at the lower 

 portion by a double layer of mesodermal cells, with a small 

 cavity between. The small cavities are the beginnings of the 

 sections of the body-cavity, found complete in the adult 

 earthworm. The cavity at the center of all is the archenteron 

 (Fig. 105, F, ar\ Fig. 106, B, ar), which means old or primi- 

 tive intestine. The mouth of the gastrula, which is called 

 the blastopore, becomes the mouth of the young earthworm. 

 At the opposite point a small opening appears ; this becomes 

 the anus of the young animal. Fig. 106, E, shows how a lon- 

 gitudinal section through a young earthworm would appear. 

 Development of the embryo to the form of the young is direct. 



As the body-cavity increases in relative size the inner 

 layer of cells of the mesoderm becomes applied closely to the 

 wall of the intestine (Fig. 106, E, al) ; the outer layer of the 

 mesoderm becomes, from that time on, a part of the outer 

 wall of the body-cavity (Fig. 106, E, m 2 ). Fig. 106, C, D, n 



