46 ZOOLOGY 



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 (ccelom). 

 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 

 from the simple condition found in the gastrula (archente- 

 ron). What is the fate of the blastopore? How does the 

 permanent mouth originate? 



6. How do males and females compare as to the number of 

 chromosomes in their sex or tissue cells ? Where the male has 

 (as is frequently the case) an odd number of chromosomes, 

 what happens in the reduction division during sperm-formation ? 

 Explain the condition indicated by the terms "x" and "y" 

 chromosomes. 



7. When there are two kinds of sperm, differing in the num- 



