REPRODUCTION 93 



shoe shape. The further process of development is 

 too complicated to be given here in detail. All 

 that is necessary to point out is the fact that the 

 original mouth cavity is divided onoe more. Two 

 folds (the coelom folds) grow from the back of the 

 gastrula into the primitive mouth cavity, dividing it 

 into two side or crelom cavities and one central narrow 

 cavity. The two side cavities ultimately become the 

 combined chest and abdominal cavities, right and left ; 

 while the narrow central part forms, later on, the gut. 

 By a process of further growth and foldings we finally 

 get all the different structures and organs of the body. 

 It will be seen that the organism passes during these 

 stages through a number of forms quite unlike its 

 mature state; in this stage it is called the embryo. 

 It has been seen that the embryo develops by division 

 from the original mother cell. Some ova are provided 

 for this purpose with a special nourishing substance, 

 the yolk, which in certain cases reaches enormous 

 proportions, the actual mother cell, here called the 

 germinal vesicle, being only microscopical (Fig. 36). 



Among the lower animals the fertilized eggs are 

 hatched outside the body, and, as explained above, 

 undergo a number of embryonic changes. In certain 

 species, such as crabs, insects, amphibians, the em- 

 bryonic stages are very distinct from each other; 

 the embryos are able to shift for themselves and 

 behave to a certain extent like adult animals. In 

 these cases the development of the embryo is spoken 

 of as its metamorphosis. To take the case of insects: 

 here the female lays her fertilized eggs on to some 

 plant, or (it may be) animal matter, to which they are 

 sometimes attached by some adhesive covering. 



