The, Microscope. 17 



Apparently closely connected with this fission is a process 

 known as conjugation, though the physiology of the act is rather 

 obscure. Two individuals will come together and unite, at times 

 being completely fused together. Then they separate. The 

 probable interpretation of this act will be apparent further on. 

 The result of the fission in the great majority of Protozoa is to 

 form two new individuals, but in some the resulting cells remain 

 attached to each other, and a compound animal is the result. 



In all other animals reproduction by eggs is the rule, 

 although in the sponges, ccelenterates, worms, etc., a true fission 

 occurs. What is an egg? Most people have a general idea on 

 this subject, but since their opinions are founded upon that most 

 familiar object, the egg of the common hen, their ideas are 

 rather confused. The egg of the common fowl contains all the 

 essential elements of the typical egg, plus a good deal more. 

 Let us for a moment consider how an egg is formed. In all the 

 Met azoa certain tissues have reproductive functions, and from 

 them arises, in the female the egg, in the male the spermato- 

 zoan. These genital tissues are made up of cells; at an early 

 stage the cells are all alike, but with growth and approach to 

 maturity certain ot these cells become greatly larger than their 

 associates, and these are destined to become eggs. By the aid 

 of a microscope we can distinguish no important differences be- 

 tween these eggs and the surrounding cells, but there is a vast 

 difference physiologically. Each of these eggs when fertilized 

 and placed under the proper conditions possesses the power to 

 reproduce the being from which it arose. 



In structure the only parts which are common to all eggs 

 are those which at the same time are common to all cells. It 

 is a mass of naked protoplasm, with a nucleus, and in this a 

 nucleolus. To these various other portions may be added. Thus 

 the yolk may be enveloped in a delicate membrane (vitelline 

 membrane) formed by the protoplasm of the egg, or hj a cho- 

 rion, usually a much thicker membrane, secreted by the follicle 

 cells in which the egg arose. Secondary envelopes like the 

 shell of the eggs of birds and reptiles may also be present, but 

 as will readily be seen they are not all important, being merely 

 protective, and taking no part in the development. 



Another adventitious element which is frequently present, 



