38 ZOOLOGY. 



58. Summary. 



1. All the higher animals begin life as a single cell and 

 reach their adult condition by a continuous series of divisions. 

 By the growth and specialization of the cells arising from 

 these divisions the great complexity of the adult body is pro- 

 duced. 



2. This initial cell the fertilized ovum represents the 

 fusion of two independent and unlike cells: the ovum (fe- 

 male) and the spermatozoon (male). 



3. Before the union (fertilization) occurs, the ovum re- 

 duces its nuclear material, by two successive divisions, to one- 

 fourth its original amount and the chromosomes to one-half 

 their original number, without a corresponding reduction of 

 the cytoplasm. The spermatozoon in its development seems 

 to undergo a similar reduction of chromosomes. 



4. After the union of the male and female cells the fertil- 

 ized ovum divides rapidly (segmentation or cleavage) form- 

 ing a mass of cohering cells. The nature of these cells and 

 of the mass depends much on the amount of yolk in the ovum 

 and on its distribution. 



5. By processes which differ in different animals accord- 

 ing to the nature of the segmentation, the cells become ar- 

 ranged with a layer outside (ectoderm), a layer within (ento- 

 derm), and from these a third layer or mass of cells lying be- 

 tween the other two (mesoderm). The entoderm bounds a 

 cavity (archenteron) which communicates by a pore (blasto- 

 pore) with the outside world. Within the mesoderm may 

 be found a cavity (ccelom). 



6. The ectoderm gives rise to the outer portions of the 

 skin, its protective and sensory structures, to the nervous sys- 

 tem, and frequently to the lining of the openings into the 

 body. The entoderm lines the principal part of the digestive 

 tract. The mesoderm gives rise to most of the other struct- 

 ures of the body. 



59. Suggestive Topics for Library Work. 



I. What suggestions have been offered as to the advantage 



