54 Animal Life and Intelligence. 



opening by which the primitive digestive tube communi- 

 cated with the exterior has during these changes closed 

 up, so that the digestive cavity does not any longer 

 communicate in any way with the exterior. This is 

 remedied by the formation of a special depression or pit 

 at the front end for the mouth, and a similar pit at the 

 hinder end.* These pits then open into the canal, and 

 communications with the exterior are thus established. 

 The lungs and liver are formed as special outgrowths from 

 the digestive tube. The ovaries or testes make their 

 appearance at a very early period as ridges of the middle 

 layer projecting into the body-cavity. For some time it 

 is impossible to say whether they will produce sperms or 

 ova ; and it is said that in many cases they pass through 

 a stage in which one portion has the special sperm-pro- 

 ducing, and another the special ovum-producing, structure. 

 But eventually one or other prevails, and the organs 

 become either ovaries or testes. 



Thus from the outer layer of the primitive embryo is 

 produced the outer skin, together with the hairs, scales, 

 or feathers which it carries ; from it also is produced the 

 nervous system, and the end-organs of the special senses. 

 From the inner layer is formed the digestive lining of the 

 alimentary tube and the glands connected therewith ; from 

 it also the primitive axial support of the body. But this 

 primitive support gives place to the vertebral column 

 formed round the notochord; and this is of mid-layer 

 origin. Out of the middle layer are fashioned the muscles 

 and framework of the body ; out of it, too, the heart and 

 reproductive organs. The tissues of many of the organs 

 are cunningly woven out of cells from all three layers. 

 The lens of the eye, for example, is a little piece of the 

 outer layer pinched off and rendered transparent. The' 

 retina of that organ is an outgrowth from the brain, which, 



* In technical language, the opening by which the primitive digestive 

 cavity (or mesenterori) communicates with the exterior is called the blastopore. 

 When this closes, the new opening for the mouth is called the stomodceum; 

 that for the vent, the proctodceum. 



