INTRODUCTION 11 



of the arcbenteron, as it occurs in the Chsetognatha, Brachio- 

 poda, and Chordonia, consists in the development of paired 

 sac-like diverticula of the archenteron, which become 

 constricted off, and then as independent coelomic sacs give 

 rise to the systems of organs of the mesoderm. Different 

 as these two kinds of mesoderm formation appear to be, 

 they nevertheless can be reduced, like the processes of 

 gastrulation by invagination and by polar ingression already 

 described, to a uniform plan, if we assume that in the first 

 case the mesodermic elements at an eai'ly period abandon 

 (as primitive mesoderm cells) epithelial continuity with the 

 entoderm, whereas in the second case the mesodermic cell- 

 mass retains provisionally its epithelial continuity, and only 

 later becomes separated from the entoderm by the formation 

 of the diverticula. 



As regards the subsequent fate of the mesoderm, we can, 

 if we disi'egard the formation of the individual organs, 

 distinguish two types. In the one case the union of the 

 mesodermic elements is loosened, and these distribute them- 

 selves in the manner of amoeboid wandering cells in the 

 space of the primitive body cavity, which eventually they 

 completely fill with a tissue consisting of stellate migratory 

 cells embedded in a gelatinous stroma. This tissue is 

 known as mesenchijma (0.. and R. Hertwig). By separation 

 of the cells of the mesenchymatous tissue, spaces (lacunne) 

 may be formed in it, which may coalesce to form lai^ger 

 spaces, and so apparently represent a kind of body cavity. 

 To such spaces the name of pseudoccele is given. 



In other cases the laTgest part of the mesoderm is employed 

 in the formation of paii'ed sacs, the ccelomic sacs, the walls 

 of which consist of a continuous epithelium of mesoderm 

 cells. The cavities contained in them represent the true 

 body cavity or coelom. The paired coelomic sacs entirely 

 surround the intestinal canal, so that the walls of the sacs 

 come together in the middle line above and below the intes- 

 tine to form the so-called mesenteries. The body cavity 

 divides the mesoderm into two layers. The outer layer, the 

 one applied to the ectoderm, is known as the somatic layer, the 

 inner one, applied to the entoderm, as the splanchnic layer. 



