222 FERTILIZATION OF THE OVUM 



stance is contained not in the nucleus but in the cytoplasm — more 

 specifically, in the case of spermatozoids, in the cilia-bearing body 

 derived from the blepharoplast, which in its development so strongly 

 suggests a centrosome (p. 172). Webber's and Ikeno's observations 

 on the cycads are not necessarily fatal to this view; for, as I have 

 shown (p. 188). the middle-piece in the echinoderm is likewise cast 

 off and degenerates near the periphery of the egg, and the centro- 

 some is a body far more minute. The possibility has been admitted 

 that this centrosome may be formed dc novo under the influence of 

 the middle-piece, which itself perishes. In like manner it may also 

 be possible that the primary stimulus in ZoDuia and like cases is given 

 by the cilia-bearing body, even though this body itself disappears and 

 the mitotic apparatus is not formed until long afterward. 



E. Conjugation in Unicellular Forms 



The conjugation of unicellular organisms possesses a peculiar inter- 

 est, since it is undoubtedly a prototype of the union of germ-cells 

 in the multicellular forms. Biitschli and Minot long ago maintained 

 that cell-divisions tend to run in cycles, each of which begins and 

 ends with an act of conjugation. In the higher forms the cells pro- 

 duced in each cycle cohere to form the multicellular body ; in the 

 unicellular forms the cells separate as distinct individuals, but those 

 belonging to one cycle are collectively comparable with the multi- 

 cellular body. The validity of this comparison, in a morphological 

 sense, is generally admitted.^ No process of conjugation, it is true, is 

 known to occur in many unicellular and in some multicellular forms, 

 and the cyclical character of cell-division still remains s?ib judice?' 

 It is none the less certain that a key to the fertilization of higher 

 forms must be sought in the conjugation of unicellular organisms. 



The dif^culties of observation are, however, so great that we are 

 as yet acquainted with only the outlines of the process, and have still 

 no very clear idea of its finer details or its physiological meaning. 

 The phenomena have been most closely followed in the Infusoria by 

 Biitschli, Engelmann, Maupas, and Richard Hertwig, though many 

 valuable observations on the conjugation of unicellular plants have 

 been made by De Bary, Schmitz, Klebahn, and Overton. All these 

 observers have reached the same general result as that attained 

 through study of the fertilization of the ^gg ; namely, that an essen- 

 tial phenomenon of conjugation is a iniioji of the nuclei of the conju- 

 gating cells. Among the unicellular plants both the cell-bodies and 

 the nuclei completely fuse. Among animals this may occur ; but in 



r/p. 58. 2^/ p. 178. 



