DEVELOPMENT 267 



anus, is formed so that the enteric pouch now communicates with 

 the exterior at both ends and becomes the alimentary canal. Thus 

 antero-posterior differentiation is clearly established. 



A cross section perpendicular to the main axis of the developing 

 worm at this stage presents the appearance of a circle within a 

 circle. The smaller circle surrounds the enteric cavity and is the 

 wall of the alimentary canal. It is separated by a space, the coelom, 

 from the larger circle, or body wall. Moreover, each of these 

 circles is composed of two tissue layers: the alimentary canal, 

 formed internally of endoderm and externally of splanchnic meso- 

 derm ; and the body wall, internally of somatic mesoderm and ex- 

 ternally of ectoderm. Thus the coelomic cavity is entirely enclosed 

 by mesoderm. (Fig. 173, K.) 



It is from these three primary layers of cells (ectoderm, somatic 

 and splanchnic mesoderm, and endoderm) that all of the tissues 

 and organs of the adult worm arise through later differentiation, 

 thickenings, foldings, outgrowths, etc. For example, the nervous 

 system is formed by the ingrowth of a thickened region of the 

 ectoderm; the blood vascular system develops by a specialization 

 of cells throughout the mesoderm; while the reproductive system 

 first appears as thickenings of the somatic mesoderm which, as 

 development proceeds, become largely separated from it as inde- 

 pendent organs in the coelom. (Figs. 60, 61.) 



B. Embryology and Metamorphosis of the Frog 



As an example of Vertebrate development we may take that of 

 the Frog, though it must be borne in mind that just as other 

 Invertebrates differ in their embryology from the Earthworm, so 

 the embryology of other Vertebrates departs widely from that of 

 the Frog — chiefly fundamental and highly significant similarities 

 persisting. 



The fertilized egg of the Frog contains a large amount of stored 

 food material, or yolk, which influences the character of the cleav- 

 age of the egg. Thus cell division is progressively more rapid, and 

 accordingly the cells smaller, in the region with little yolk, the 

 upper (animal) pole, than in the yolk-laden lower (vegetal) pole. 

 The first and second division planes are from pole to pole, and 

 give rise to four cells of equal size. The third division plane 

 is just above the equator of the egg, at right angles to the former 

 planes, and establishes four dark, pigmented cells above, and four 



