DEVELOPMENT 265 



While these two primary germ layers are being established, the 

 developing embryo shows the rudiments of the third primary germ 

 layer (mesoderm) in the form of two mesoblast cells which 

 leave their original position in the wall of the embryo and take 

 up a place between the ectoderm and endoderm; that is, in the 

 remnant of the cavity of the blastula which the invagination proc- 

 ess during gastrulation has not completely obliterated. Here the 

 two cells, by division, form on either side of the enteric pouch a 

 linear series, or band, of mesoderm cells. These mesoderm bands 

 gradually increase in size and spread out until finally they unite 

 above and below, that is encircle, the enteric pouch. Thus they 

 form a continuous mesoderm layer between ectoderm and endo- 

 derm. Simultaneously with the growth of the mesoderm bands to 

 form a definite middle layer, a linear series of spaces appears in 

 each band which presages the future segmentation of the worm's 

 bodv. These cavities increase in size and, when the bands unite 

 around the enteric pouch, the corresponding cavities of each band 

 also become continuous in the same regions. (Fig. 173, C-H.) 



In this way the mesoderm itself becomes divided into what are 

 essentially two cellular layers, an outer, or somatic layer, next 

 to the ectoderm, and an inner, or splanchnic layer, in contact 

 with the endoderm. The space between these layers of the meso- 

 derm is the body cavity, or coelom. The coelom, however, is not 

 a continuous cavity from one end of the embryo to the other, be- 

 cause the mesodermal cells which separate the linear series of 

 cavities in the respective mesodermal bands persist. These cells 

 form a regular series of connecting sheets of tissue between the 

 somatic and splanchnic mesoderm layers and thus divide the body 

 of the worm into a series of essentially similar segments, the limits 

 of which are indicated on the outside by a series of grooves which 

 encircle the worm's body. (Fig. 173, I, J.) 



While these processes are transforming the two-layered gastrula 

 into an embryo composed of three primary layers, and exhibiting 

 segmentation, coelom, etc., — in short, the 'tube within a tube' 

 body-plan characteristic of higher forms — the embryo is gradu- 

 ally increasing in size and elongating. The mouth, representing 

 the blastopore, remains at one end, which is therefore designated 

 as anterior, while growth is chiefly in the opposite direction or 

 toward the posterior. At this end (the blind end of the enteric 

 pouch formed at gastrulation) an opening to the exterior, the 



