THE SCAFFOLDING LEFT IN THE BODY 109 



arches and dyed scarlet with the blood which con- 

 tinually courses through them. In many fishes 

 these arches are five or seven in number, and 

 communicating with them — in order to allow the 

 aerated water, which has been taken in at the 

 mouth, to pass out again after bathing the gills — 

 an equal number of slits or openings is provided 

 in the neck. Sometimes the slits are bare and 

 open so that they are easily seen on the fish's 

 neck — anyone who looks at a shark will see 

 them — but in modern forms they are generally 

 covered by the operculum or lid. Without these 

 holes in their neck all fishes would instantly perish, 

 and we may be sure Nature took exceptional care 

 in perfecting this particular piece of the mechanism. 

 Now it is one of the most extraordinary facts 

 in natural history that these slits in the fish's neck 

 are still represented in the neck of Man. Almost 

 the most prominent feature, indeed, after the head, 

 in every mammalian embryo, are the four clefts or 

 furrows of the old gill-slits. They are still known 

 in Embryology by the old name — gill-slits — and so 

 persistent are these characters that children are 

 known to have been born with them not only ex- 

 ternally visible — which is a common occurrence — 

 but open through and through, so that fluids taken 

 in at the mouth could pass through and trickle 



