DEVELOPMENT OF THE FERTILIZED EGG 285 



of nearly all animals. Among some of the very lowest ( Hydra} 

 only the ectoderm and the endoderm are formed, the mesoderm 

 being omitted. But in all except the lowest types three layers 

 are formed early in the embryological history. The method 

 by which these three layers are formed differs in different ani- 

 mals. In Figure 15 is shown a method of formation of the 

 endoderm, differing from that of the frog, by an infolding of a 

 hollow sphere to form a double sac. But however differently 

 the layers are formed, the system of organs which are developed 

 from them is essentially the same. The nervous system is 

 always developed from the ectoderm, the alimentary canal 

 from the endoderm, and the blood system and muscles are 

 developed from the mesoderm. 



4. The Formation of the Body. While the germ layers have 

 been forming, the embryo has been elongating (Fig. 132 E), and 

 the endoderm forms itself into a hollow tube within the body, 

 which acquires an opening, first at one extremity and then at 

 the other; Fig. G. This tube becomes the alimentary canal, 

 and the two openings are the mouth and the anal, or cloacal 

 opening. Between this inner tube and the outer wall of the 

 body lies a cavity, more or less filled with the mesoderm, but 

 in it eventually appears the body cavity or coelum, which be- 

 comes a more distinct cavity as the animal grows. Early in 

 the development, when the animal has assumed the form shown 

 at E, openings in the side of the neck break through from the 

 alimentary canal to the exterior. There are at first two of these, 

 shown at E, brc, but later others appear. These are known 

 as branchial openings, and become passages through which 

 water taken in at the mouth may be passed to the exterior. 

 They represent the gill slits present in fishes, and are to 

 have a similar function a little later, when the frog hatches 

 from the egg and lives in the water. While these changes are 

 going on there is formed a long, thickened rod of ectoderm in 

 the middle line of the back, extending from one end of the ani- 

 mal to the other, which is the beginning of the nervous system; 



