SYNGAMY 245 



One should not, however, lose sight of the fact that cytoplasm and 

 chondriosomes are passed from generation to generation through the 

 egg. As will be pointed out later, much that happens in the embryo 

 depends upon the constitution of the egg cytoplasm. That which under- 

 goes development is a complete protoplast; no differentiation occurs 

 without the combination of nucleus and cytoplasm, both of which are 

 derived directly from the preceding generation through one or both of the 

 gametes. The presence of chondriosomes in the egg and sometimes in 

 the sperm probably indicates the occurrence of certain physiological 

 processes as it does in other cells. Such processes may influence the 

 course of differentiation, but there is as yet no adequate evidence that 

 the chondriosomes act as differential factors of inheritance in any way 

 comparable to the action of nuclear factors. 



Conjugation and Endomixis. — From among the many kinds of 

 syngamic phenomena in Protozoa^*' a single peculiar example is selected 

 for description here. Conjugation in Paramoecium, as originally shown 

 by Maupas (1889), involves a complicated series of nuclear changes. 

 Wilson (1900) describes these as follows: 



... in Paramoecium 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 mac- 

 ronuclei 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. 

 caudalum 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. 



It was supposed for a long time that conjugation involving an inter- 

 mixture of nuclear material from two individuals was necessary for the 



^See M. Hartmann (1909, 1914), Erdmann (1920), G. Hertwig (1921), and 

 especially Jennings (1923), Belaf (1922), Kofoid (1923), E. B. Wilson (1925), and 

 Calkins (1926). For a diagram of conjugation in Paramcecium, see Morgan (1913, 

 p. 6). Conjugation in Dileptus is described by Visscher (1927). 



