374 AN INTRODUCTION TO ZOOLOGY 



The ova of different species vary in the amount and 

 distribution of food material they contain, and also in the way in 

 which they may be protected by the development of enclosing 

 membranes. While in the ovary the egg is contained within a 

 follicle which may fit closely round it, or may enlarge and become 

 vesicular as in the Mammal, when the egg with a certain number of 

 cells surrounding it lie to one side. While in the ovary two enclosing 

 egg membranes may arise. Firstly, there is an envelope actually 

 secreted by the ovum itself ; this is termed the primary egg mem- 

 brane, or vitelline membrane, and is usually very thin and structure- 

 less, but may be thicker and perforated by minute radial pores, 

 when it is designated the zona radiata. In some cases it may even 

 consist of an internal zona and an external clear membrane. 



A secondary or follicular membrane may also be developed, but 

 this is a product of the follicular cells, and not of the ovum itself. 

 In many animals it is hard to distinguish how much of the envelope 

 around the ovum in the ovary is primary and how much is secondary. 

 Generally, in the Insects and Bony Fishes, the follicular membrane 

 is quite distinct, and termed the " chorion." Often these two 

 membranes are perforated at one point by a tiny canal, the micropyle, 

 which allows the sperms to reach the ovum. 



When the ovum is ready it escapes from the ovary by the actual 

 rupture of the follicle, a process that is termed ovulation. It makes 

 its way to the oviducal funnel, and as it passes down the oviduct it 

 may have deposited around it, as the result of the activity of the 

 oviducal glands, one or more tertiary egg membranes. These may 

 consist of a jelly-like mucilaginous substance as in Rana, or an 

 albuminous layer as in ScylUum and the fowl, and in the case of 

 these two animals, the dogfish has, in addition, a chitinous case, 

 while the fowl has a calcareous shell. The provision of any or all 

 of these membranes and their nature varies to meet the requirements 

 of the conditions under which the egg is laid and develops. 



Finally, the ovum itself may be provided with a large or small 

 amount of food material, the deutoplasm, usually in the form of 

 tiny granules, the yolk spheres. In certain animals, e.g. Amphioxus 

 and the higher mammals, this supply is quite small and fairly evenly 

 distributed throughout the cytoplasm of the ovum, and such an egg 

 is termed homolecithal. On the other hand, and more commonly, 

 there is a large amount of yolk present, and as it is unevenly dis- 

 tributed the ova are termed heterolecithal. In some forms, like 

 Insects and Crustacea, it is concentrated towards the central part 

 of the egg, leaving the peripheral protoplasm more or less free, a 

 condition known as centrolecithal. The dogfish and the fowl have 

 eggs with an enormous amount of yolk that is concentrated all at 



