CHAPTER XL 



THE EGG, THE SPERM, AND THE DEVELOPMENT OF. 

 THE CHICK. 



Egg. 

 Th?: fowl's egg, in spite of its size, is a single cell, just as is the 

 microscopic egg of the earthworm. It is wrapped, however, 

 in an adventitious sheath, derived from the walls of the 

 passages through which it makes its way to the exterior when 

 "laid." The real egg — the term oviim is preferable, as not 

 implying the non-essential coverings — is limited to that part of 

 the egg which is popularly called the yolk. All outside of the 

 delicate membrane covering the yolk is the adventitious 

 sheath. This sheath consists of the "white," or albumen, 

 which is fluid in the fresh egg, has two spiral thickenings at 

 each pole, the so-called " chalazse," the use of which is, 

 apparently, to act as springs to prevent the delicate ovum itself 

 from being jarred and ruptured. Outside of the albumen is 

 a tough membrane, and outside that, again, the shell. The 

 albumen is absorbed by the growing chick; the numerous 

 pores in the shell permit of respiration being carried on before 

 the young chick breaks through the eggshell at hatching. The 

 ovum proper is enclosed by a delicate and elastic vitelline 

 membrane. The substance of the ovum is chiefly made up of 

 the yolk ; but there is a cap of pure protoplasm at one side, in 

 which lies the nucleus or germiiial vesicle. The great size of the 

 ovum is due to the enormous quantity of this yolk present. 



Ova are classified according to the quantity of yolk present 

 and its distribution. There are three grades, connected, of 

 course, by intermediate conditions. 



