88 HOW ANIMALS DEVELOP 



down and an idea of the way in which this funda- 

 mental pattern is called forth. There are many 

 excellent books, such as MacBride's Invertebrate 

 Embryology and Graham Kerr's Vertebrate Embryology, 

 where the details of the later processes of develop- 

 ment are described. The reader who consults these 

 books will be amazed at the mass of knowledge we 

 have about the development of the embryos in 

 different animal groups. 



It has been said above that, as a rule, embryos of 

 different kinds resemble one another the more the 

 younger they are. This is not always true as regards 

 the separate organs. It is very interesting to find that 

 sometimes distantly related animals have evolved 

 similar organs, but have evolved them in quite dif- 

 ferent ways which are recapitulated in their embryo- 

 logical development. The clearest examples of this 

 are seen in organs which have to perform some quite 

 definite function, which more or less dictates their 

 structure. For instance, an efficient eye must have a 

 sensitive layer of tissue (the retina) to perceive light, 

 a lens to focus the light, and an apparatus like an 

 iris-diaphragm on a camera to regulate the amount 

 of light which enters. In the higher vertebrates, 

 such as man, mammals, birds, and newts, the retina 

 originates from an outgrowth of the neural plate in 

 the brain region which we have already described. It 

 induces the formation of a lens from the skin of the 

 head. The iris is formed from the retina (Fig. 21). 

 Now all these parts are quite well-developed in the 

 eyes of octopuses, which belong to the group of 



