20 HOW ANIMALS DEVELOP 



vertebrate occurs before the stages representing the 

 various groups which gradually evolved out of the 

 original vertebrate stock. This hypothesis brings 

 under one head a large number of very odd facts. 

 For instance, a young mammalian embryo, such as 

 a young human embryo about four or five weeks 

 old, is provided with gill slits and blood-vessels which 

 flow along them. These are like the organs found in 

 fish, where the blood flows through the gills and 

 absorbs oxygen from the water, but they can be of no 

 possible use to a mammalian embryo, which at this 

 stage is deriving its oxygen from the blood-stream 

 of its mother. Haeckel's theory, that such organs are 

 * 'hang-overs" from the time when the ancestors of 

 mammals were fish, still provides the most con- 

 venient way of describing this whole class of 

 phenomena. But we have slightly modified the 

 expression of Haeckel's theory. Many details of 

 embryonic development are better described as 

 reflections not of adult ancestors, as Haeckel thought, 

 but rather of the embryonic stages of those ancestors. 

 The mammalian embryo has gill slits, not like the 

 gills its ancestors had when they were adult, but 

 like the gill slits they had when they were embryos. 

 With this modification, Haeckel's hypothesis, 'the 

 so-called "biogenetic law" or recapitulation hypo- 

 thesis, is still one of the foundations of our system of 

 descriptive embryology. 



But even so, there are very many features of 

 development to which the law does not apply. Many 

 embryonic characteristics do not represent any 



