THE GERM WALL. 



229 



On either side of the germ wall there is a sharp distinction between the defini- 

 tive ectoderm and mesoderm on the one hand, and the single layer of columnar 

 blastoderm cells on the other. (Fig. 129.) 



It thus appears that in Limulus the formation of germ layers is not a 

 particular event in the development. It takes place at different times and 

 in widely different parts of the germinal area. In fact, the production of 

 germ layers may occur wherever early growth takes place, as for example, 

 on the sides of the anal plate, where the mesoblastic somites are formed; in the 

 germ wall, where the germ layers in the lateral ends of each half metamere are 



FIG. 129. Section through the primitive streak of Limulus, stage G, showing mesoblastic somites, germ wall, 



periblast, etc. 

 FIG. 130. Sections of same stage through posterior parts of the anal plate. 



formed; in the telopore, where the axial tissues arise; or in the comparatively 

 late stages of development, where mesodermic tissues appear to arise from various 

 local modifications of the ectoderm. The formation of germ layers is not, there- 

 fore, synonymous with gastrulation, nor is it to be regarded as a modification of 

 gastrulation, which, as we understand it, is that particular process by which an 

 organ representing the enteron of a ccelenterate was formed; as for example, the 

 central ingrowth of the primitive cumulus, and of the procephalic lobes. The 

 formation of the germ layers from the telopore and germ walls, is ontogenetically 

 and phylogenetically a later and a different process. It is the embryonic method 

 of growing a new body on an old head, the head representing the ancestral ccelen- 

 terate body. Neither the embryonic method of postcephalic growth in segmented 

 animals, nor the great variety of tissues and organs produced by it, are comparable 

 with anything that takes place in the coelenterate. 



