FROM SIMPLE CELL TO COMPLEX ANIMAL 45 



original number, without a corresponding reduction of the cyto- 

 plasm. The spermatozoon in its development undergoes a 

 similar reduction of chromosomes. 



4. After the union of the male and female cells the fertilized 

 ovum divides rapidly (segmentation or cleavage) forming 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 according 

 to the nature of the segmentation, the cells become arranged 

 with a layer outside (ectoderm), a layer within (entoderm), and 

 from these a third layer or mass of cells lying between the other 

 two (mesoderm). The entoderm bounds a cavity (archenteron) 

 which communicates by a pore (blastopore) with the outside 

 world. Within the mesoderm may be found a cavity (coelom). 



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

 its protective and sensory structures, to the nervous system, 

 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 structures of the body. 



62. Suggestive Topics for Library Work. 



1. What suggestions have been offered as to the advantage 

 of the addition of the male nucleus to that of the female in 

 fertilization? Has a similar result ever been attained arti- 

 ficially by means of chemical or other stimuli ? 



2. What explanations have been offered as to the signifi- 

 cance of the process of maturation? Trace the maturation 

 of the sperm cells more fully. 



3. What classification of ova do the text-books make? 

 What is the basis of the classification? To what extent do 

 eggs of different animals vary in size, shape, envelopes, etc. ? 

 Give examples. 



4. Is there any explanation of the fact that there is such 

 a difference in the amount of food substance in the eggs of 

 different animals ? 



5. Trace out by reference to a text-book of embryology the 

 principal changes by which the adult digestive tract is derived 



