THE HUMAN ORGANISM 



28 



227. Development. The transformations observed during the de- 

 velopment of insects (metamorphosis, see page 29) are matched by 

 equally great changes in the development of backboned animals. For 

 most of the familiar vertebrates this process is concealed from us be- 

 cause it takes place within the body of the mother or, among birds and 

 reptiles, inside the egg. In fishes and amphibians, however, the stages 

 can be easily seen, as the development takes place in the water (see Fig. 

 138). Since every individual begins life as a single cell, this stage has 

 been compared to a protozoon. This one cell, or fertilized egg, divides 

 into two ; each of these divides again ; and so on. For a considerable part 

 of the journey from a one-celled organism to a many-celled organism all 

 the higher species are very much alike (see Fig. 107). When we com- 

 pare the embryos of several kinds of backboned animals, as the fish, the 



a 





Fig. 138. Development of the frog {Rana palustris) 



a, eggs; b, newly hatched tadpole; c, older tadpole with gills and front legs; d, tad- 

 pole with hind legs; e, young adult (gills and tail absorbed) and older mature frog 



bird, the salamander, and the rabbit, we find that they are ver>' much 

 alike early in their development, not only when each consists of a single 

 cell but even later, when it is possible to distinguish head and trunk and 

 limbs (see Fig. 139). As they become older they differ from each other 

 more and more. During the development of the human organism after 

 birth the different parts of the body do not grow at the same rate (see 

 Fig. 140). The bones of the skull in the newborn infant do not quite 

 meet at the edges (Fig. 141). 



228. Hygiene of the skeleton. The normal growth and devel- 

 opment of the skeleton depend upon nutritive conditions dur- 

 ing infancy. Defects in the food related to bone formation are 

 of two kinds: (i) improper proportions or quantities of cal- 

 cium and phosphorus and ( 2 ) lack of vitamin A. Normal milk 

 ordinarily yields both the needed minerals and the vitamin A, 



