8 



SMITHSONIAN AnSCELLANEOUS COLLECTIONS 



vor 



8 1 



layers (fig. 6,Msd). In the annelids, the coelomic sacs form the 

 entire segmented body cavity ; in Pcripatus and most of the arthropods, 

 the greater part of the definitive body cavity is derived from a space 

 between the ectoderm and the endoderm lined with mesenchymatic 

 cells. 



It is most important to bear in mind the intimate relation that exists 

 between the mesoderm and the endoderm. In the arthropods, especi- 

 ally in insects, the process of gastrulation, as above noted, is greatly 

 modified, and mesoderm tissue alone is proliferated along the greater 

 part of the length of the blastopore area, which in only a few general- 

 ized forms appears as a true opening. At each end of the mesoderm, 

 however, endodermal tissue is formed (fig. 5 C, AMR), and the two 



~-M5d- 



FiG. 6. — Formation of mesoderm in Peripatiis cafciisis. (From Balfour, 1883.) 



Cross sections of embryos through blastopore, showing formation of meso- 

 dermic coelomic sacs (Msd) from endoderm (End) just within lips of blasto- 

 pore (Bp). 



endoderm rudiments mark the anterior and the posterior limits of 

 the mesoderm — consequently, they define the area of segmentation. 



It is unnecessary to speculate as to the phylogenetic steps that may 

 have led from the early creeping gastrula form of animal to the worm- 

 like ancestor of the arthropods, but we must note the important 

 advance in cephalization, and the possibilities of further head develop- 

 ment that came with the establishment of a mouth at the anterior end 

 of the body. Food, whether living or inert, had now to be recognized 

 and seized on contact. Consequently, it became highly important to 

 the animal to be able to determine its course according to favorable 

 or unfavorable conditions of the surroundings. The ectoderm of the 

 anterior end of the body developed a special sensitiveness to environ- 

 mental changes, and, probably by means of ectodermal processes ex- 

 tending into the body, communicated the stimuli received from with- 

 out to the internal tissues. Certain groups of the sensitive cells then 



