The Microscope. 107 



OUTLINES OF EMBRYOLOGY. 



FOURTH PAPER. 



Metameric Segmentation. 



ALMOST all animals above the Protozoa may be readily 

 divided into two groups ; in the first the body consists of a 

 series of somewhat similar segments arranged one after another 

 in serial order, while in the other no traces of this segmentation 

 occur. In the Ouvierian system of classification there was rec- 

 ognized a group of animals, under the name Articulata, the 

 fundamental character of which was this segmentation of the 

 body. Later studies have shown that this segmentation or 

 metamerism is not confined to the articulates, but occurs in 

 other groups. 



To understand better the extent of this metamerism, let us 

 examine one of the animals in which it is best developed ; for 

 instance, an angle-worm. Externally, we see that the body is 

 composed of a long series of rings arranged one after another, 

 rendering the segmentation of the body apparent to the most 

 casual observer. With a low power (or even with the naked 

 eye) we can see on each segment a pair of bristles used in place 

 of legs. When we dissect the worm we find that this same 

 segmentation occurs in almost every organ. The nervous sys- 

 tem has a ganglion in each segment, the muscles are correspond- 

 ingly arranged, there is a pair of excretory organs in each ring, 

 the heart extends through all, as also does the digestive tract, 

 while the reproductive organs alone are confined to one portion 

 of the body, and even these exhibit a tendency toward meta- 

 meric repetition. Each ring of the worm contains a portion of 

 all the systems except the reproductive, and is essentially like 

 its fellows. In some nearly allied forms the reproductive or- 

 gans form no exception. 



This serial repetition of parts is also well illustrated in the 

 vertebrates, especially in the lower forms and in the lar- 

 val stages. The nervous system is composed of a number of 

 ganglia arranged one after another ; the vertebral column con- 

 sists of separate bones corresponding to the ganglia. In the 

 muscles of the trunk (especially in the fishes) the same is seen, 



