284 INTRODUCTION TO CYTOLOGY 



of Wilson (1900, pp. 224-226) In "Paramecium caudatum, which 

 possesses a single macronucleus and micronucleus, . . . conjugation is 

 temporary and fertilization mutual. The two animals become united 

 by their ventral sides and the macronucleus of each begins to degenerate, 

 while the micronucleus divides twice to form four spindle-shaped bodies. 

 Three of these degenerate, forming the ' corpuscles de rebut,' which 

 play no further part. The fourth divides into two, one of which, the 

 'female pronucleus,' remains in the body, while the other, or 'male 

 pronucleus,' passes into the other animal and fuses with the female 

 pronucleus. Each animal now contains a cleavage-nucleus equally 

 derived from both the conjugating animals, and the latter soon separate. 

 The cleavage-nucleus in each divides three times successively, and of 

 the eight resulting bodies four become macronuclei and four micronuclei. 

 By two succeeding fissions the four macronuclei are then distributed, 

 one to each of the four resulting individuals. In some other species 

 the micronuclei are equally distributed in like manner, but in P. caudatum 

 the process is more complicated, since three of them degenerate, and 

 the fourth divides twice to produce four new micronuclei. In either 

 case at the close of the process each of the conjugating individuals 

 has given rise to four descendants, each containing a macronucleus and a 

 micronucleus derived from the cleavage-nucleus. From this time forward 

 fission follows fission in the usual manner, both nuclei dividing at each 

 fission, until, after many generations, conjugation recurs." 



The Physiology of Fertilization. — The principal results of fertiliza- 

 tion are two: the activation of the egg, and, in dioecious organisms, 

 biparental heredity. Both of these have their physiological as well as 

 their morphological aspects, and in the present section the first of them 

 will be considered with special reference to its physiology. What is the 

 nature of the physiological reactions through which the development of 

 the egg is initiated? In the terms used by Child (1915), how does 

 fertilization bring about the rejuvenation of the egg, which is a physio- 

 logically old cell? The attack upon this problem has been carried out 

 along two main lines: by a study of artificial parthenogenesis and by a 

 direct analysis of the chemical constitution of the gametes at these stages. 



Artificial Parthenogenesis. — This line of attack has been followed 

 particularly by Loeb, who has found a number of methods by which the 

 parthenogenetic development of unfertilized eggs may be artificially 

 induced. 1 As stated in the foregoing pages, the first externally visible 

 effect of fertilization is in many animals the formation of a fertilization 

 membrane. The formation of this membrane, which seems to be a 

 condition necessary to the future development of the egg, Loeb was able 

 to induce in the California sea urchin by placing the eggs for 2 minutes 

 in a solution made up of 50 c.c. of sea water and 3 c.c. of a tenth-normal 

 1 For a convenient summary of such methods see Harvey (1910). 



