INTRODUCTION 901 



with the result that the number of eggs produced at each breeding period is 

 reduced enormously, compared with that of the cod or ling. 



2. General Environmental Conditions Necessary for 

 Development 



Regardless of whether or not there is specialized care of the developing 

 young, the following conditions, concerned with the nutrition and care of the 

 young, are necessary in the development of all vertebrate embryos: 



(a) All embryos develop within a fluid or "embryonic lake" made possible 

 by the presence of certain, enveloping membranes; 



(b) a favorable temperature is required, particularly in warm-blooded 

 species; 



(c) food material including water must be supplied; 



(d) oxygen is necessary to the developing embryo, and 



(e) the removal of carbon dioxide and other wastes is imperative. 



3. Types of Enveloping or Protective Membranes 



Many types of protective membranes are produced in the vertebrate group 

 for the purpose of caring for the developing young. These membranous and 

 other types of protective envelopes may be classified as follows: 



a. Egg membranes. 



b. Extra-embryonic membranes. 



c. The uterine portion of the oviduct. 



d. Uncommon or specialized structures. 



The egg membranes are those membranes produced around the egg during 

 its formation in the ovary or during the journey down the oviduct. They are 

 classified generally into three categories: 



( 1 ) Primary egg membranes are the membranes which are produced by 

 the surface layer of the egg as it develops in the ovary, e.g., the vitel- 

 line membrane; 



(2) secondary egg membranes are the membranes contributed to the egg 

 by the activities of the surrounding follicle cells of the ovary, e.g., the 

 zona pellucida of mammals, possibly also the chorion of some fish 

 eggs; and 



(3) tertiary egg membranes are the membranes contributed to the egg as 

 it passes down the oviduct, such as the albuminous layers of frog and 

 chicken eggs. 



The extra-embryonic membranes are those membranes constructed of 

 embryonic tissues which extend out of and beyond the strict confines of 

 the embryonic body. As such they represent specialized embryonic tissues 



