GENERAL EMBRYOLOGY 



147 



arises a third germ-layer, which owing to its position between the first 

 two, is called the mesoderm, mesohlast, or middle germ-layer; this naturally 

 can come only from the cell material of the existing germ-layers, indeed 

 only the entoderm seems to participate in it. Two methods can he 

 distinguished in its formation. In one the space between ectoderm and 



Fig. io8. — Larvse of Strongylocenirotus lividus (after Boveri). Left a blaslula witli 

 mesenchyme formation; right a gastrula with differentiated mesenchyme. 



entoderm becomes widened Ijy the secretion of gelatinous substance, and 

 from the entoderm isolated cells push into this jelly; thus there arises 

 a middle layer, the mesenchyme (fig. io8), somewhat similar to gelatinous 

 connective tissue, from which certain organs either wholly or in part 

 take their origin. The mesenchyme can be formed before the beginning of 

 gastrulation or after that process has been completed (fig. io8; cf. p. 74). 



Fig. ioq. — Formation of tlie mesothehum and crclom of Sagitla. A. From the 

 bottom of the gastrula arise two folds, which divide the archcnteron into the permanent 

 digestive tract and the ccelomic diverticula. B. The separation is almost completed 

 by the pushing up of the folrls. ak, outer, mk, middle, ik, inner germ-layer; nik\ 

 somatic layer; mk'-, splanchnic layer; Ih, body-cavity. 



Mesothelium. — In the second case the mesoderm may preserve the 

 epithelial character of the two primary germ-layers, and is called meso- 

 thelhmi. It is cut off from the entoderm, the mode of development being 

 shown in the worm Sagilta (fig. 109). When the gastrula of Sagitta 

 has been formed two folds arise from the archenteric walls opposite the 



