FROM SIMPLE CELL TO COMPLEX ANIMAL 41 



(i) from the multiplication of a few special cells which, before 

 invagination, in early cleavage stages, become distinct from 

 those that are to form ectoderm and entoderm (Fig. 14, A and 

 B, m)\ (2) by means of isolated, wandering cells budded from 

 the other two layers, particularly the entoderm (Fig. 14, C, c); 

 or (3) from entoderm, in the form of pouches or of solid buds 

 of cells which arise from the walls of the archenteron and extend 

 into the segmentation cavity (Fig. 15, m). In some instances 

 there may occur a combination of these methods. 



58. Coelom. When the mesoderm develops by the last 

 mentioned method, i. e., by the evagination of the wall of the 

 primitive gut (Fig. 15, m), we see a pair of folds, or a series of 

 pockets, the cavities of which are at first continuous with the 

 archenteron, but later become separate from it and entirely 

 surrounded by the mesodermic layers. The outer wall of the 

 mesodermic pouches joins the ectoderm and forms a body wall, 

 and the inner applies itself to the entodermal wall of the gut. 

 The space between is the ccelom or body cavity. When the meso- 

 derm arises as a solid mass, instead of a pocket, the body cavity 

 is formed by the splitting of the mass into an inner and an outer 

 portion. When the ccelom is formed by several pockets the 

 cavities of these may ultimately coalesce, forming a single body 

 cavity. Such a cavity is found in all the vertebrates and in the 

 higher invertebrates, although it may become more or less ob- 

 scured and modified in the adult. 



59. Differentiation of Organs and Tissues. We have al- 

 ready in these three layers and their foldings the fundamental 

 outline of that differentiation which is to give us the complex 

 animal form found in the adult. From these layers, singly or in 

 combination, all the tissues and organs of the body arise. The 

 various layers become locally thickened, folded, or otherwise 

 modified in form by rapid cell division, thus producing the begin- 

 nings of organs. At a later date differentiation takes place 

 among the cells, and tissues arise (see next chapter). In general 

 each layer gives rise to such structures as its position and relation 

 to the outer layers would suggest. This is especially noticeable 

 in the ectoderm and entoderm. The ectoderm is more closely 



