224 



nUNCIl LES OF ANIMAL JilOLOCY 



the gorin cells can l)e early distinguished. If that were possible, the 

 earliest recognizable cells whose ultimate fate were known would be re- 

 garded as the genns of those organs. In the absence of such knowledge, 

 the three layers ectoderm, endoderm, and mesoderm exhibit the first 

 general division of the embryo into parts whose destiny can be predicted 

 — hence the name germ layers. 



The development of the organs from these layers is termed organ- 

 ogeny. In very large measure the production of organs consists of the 

 rolling, or folding, or protrusion of portions of various layers of cells. 

 When a layer of cells is bent inward, into a cavity enclosed by that layer, 

 the bending is termed invagination. If the layer of cells is bent or folded 



ms 



Fig. 180. — Cross-section of the early embryo of a frog, diagrammatic, c, coelom; 

 dig, digestive tract; ee, ectoderm; en, endoderm; ms, mesoderm; nc, neural crest; nd, note- 

 chord; nf, neural fold; ng, neural groove. 



outward, the protrusion is called evagination. Invagination and evagi- 

 nation may occur in long and narrow areas, restdting in the formation of 

 furrows or ridges. Or the bending may occur in a very limited region, 

 producing pits or knobs. 



Owing to the great diversity of organs found in various groups of 

 animals, and to the fact that many of these organs have no counterpart in 

 other groups, it seems wise to confine the account of the early development 

 of organs to those of the vertebrates. Even within this one group a 

 complete list of organs cannot be mentioned. Some of the more impor- 

 tant ones will be described, to illustrate the kind of processes involved in 

 organ formation. 



