42 



General Part. 



Movable appendages^ which subserve locomotion, are called 

 limbs. They are usually elongate, and often jointed. Among the 

 lower animals they play only a small part, whilst in the Arthropods 

 and higher Vertebrates they become important as locomotor organs. 

 Of other important appendages may be mentioned the tentacles, 

 antennse, etc., of different animals, employed as tactile or prehensile 

 organs (see also under skin) . The end of an outstretched appendage, 

 farthest from the body, is called the distal, that which is nearest^ 

 the proximal, end. 



IV. Embryology or Ontogeny. 



Embryology treats of development from the egg to the 

 mature organism, that is, the changes which the individual under- 

 goes until it reaches the adult form. As a matter of fact, the animal 

 changes continually during its entire existence, and the developmental 

 history should really, therefore, embrace the whole life. Prac- 

 tically, however, it is restricted to the earlier periods, when changes, 

 are more obvious than they are later. 



A special interest attaches to the earliest stages in most. 

 Metazoa, for they exhibit striking conformity, a characteristic 

 common type, in spite of numerous individual modifications. 



1. In the simplest cases (Fig. 29), development begins with the 

 division of the ovum into two nearly equal cells, each of which 

 divides again into two, in a radial plane, so that the result of the 

 segmentation, as this process of ^ division is called, is a number o£ 



Fig-. 29. Stages in the development of the ovum of a Nemertine (Lineus) . 1 Th* 

 egg divided into two cells. 2 Tovmg blastula with small segmentation cavity. 3 — 4 

 Later stage of segmentation with larger segmentation cavity. 5 — 6 Younger and older 

 gastrul*. 1 seen from the surface, the rest are sections, cs segmentation cavity, ea 

 epiblast, en hypoblast. VT Archenteron. — After Barrois. 



