SEXUAL REPRODUCTION. I39 



In the spindle the nuclear elements, still distinguishable in their orderly 

 behavior as male and female, eventually form what is known as the "equatorial 

 plate " (VI), lying across the center of the spindle. This is a well-marked stage, 

 and one characterized by apparent equilibrium. "It is the resting-stage par 

 excellence m the life of the cell. Movement is at an end, a state of stability has 

 set in, and this would continue ad infinitum, did not a factor, which hitherto has 

 played no part, assert itself and bring about fresh movement. This new 

 movement is the longitudinal division of the chromatin elements, an independent 

 expression of life — indeed, a reproductive act — on the part of the nuclear 

 elements." 



The above short sketch will show how intricate, and yet at the 

 same time how orderly, are the intimate processes of fertilization. 

 Variations do indeed occur, both in pathological and in apparently 

 normal cases ; but a general constancy is now both clear and certain, 

 not only for many different animals, but. also to a certain extent, as 

 Strasburger has shown, for plants. 



One marvelous fact, showing the closeness of union in fertilization, 

 may be briefly re-emphasized. In the double nucleus formed from the 

 union of male and female nuclei, Van Beneden, Carnoy, and others, 

 have shown that both constituents have an equal share. "The one 

 half is purely male, the other purely female, and this is true not only 

 for Ascaris (by Van Beneden) and other threadworms (by Carnoy), 

 but for representatives of other worm-types, coelenterates, echinoderms, 

 mollusks, and tunicates." In division to form daughter-cells (IX X), 

 half of each set of constituents goes to either cell, and the dualism is 

 kept up. Furthermore, though hardly yet quite certain, it is most 

 probable, that "of the four chromatin loops observed in the division- 

 figure of a daughter-cell, two are derived from the male parent and two 

 from the female." The importance of this fact, in relation t r the 

 influence of both parents upon the offspring, is very obvious. 



V. Fertilization in Protozoa.— In the nascent sexual union observed in 

 many Protozoa, — not, however, as yet in foraminifers or radiolarians, — consider- 

 able diversity obtains. The individuals which unite may be to all appearance 

 similar (to which cases the term conjugation is generally applied), or they may 

 be materially dimorphic, as in Vorticella. The union may be permanent, when 

 the two units fuse into one ; or it may only be temporary, during which an 

 interchange of elements takes place. In both cases the nuclear elements play 

 an important part, disrupting and reconstructing during the process, while a 

 genuine fusion of the two nuclei has also been observed in permanent 

 conjugation. 



In regard to the interchange of elements, there is considerable divergence of 

 observation. Joseph has noted what appears to be an interchange of 

 protoplasm; Schneider has observed the exchange of nuclear elements ; while 

 Gruber and Maupas, and Joseph as well, have, in their studies on the union of 

 ciliated infusorians, laid emphasis on an accessory nuclear body, generally 

 known as the " paranucleus." This body lies by the side of the larger nucleus, 



