196 REDUCTION OF THE CHROMOSOMES 



mother-cell of the embryo-sac the number of divisions before fertiliza- 

 tion is three, four, five, or sometimes even more, the reduced number 

 persisting throughout. These facts led to the suspicion, first expressed 

 by Overton in 1892, that the reduced number of chromosomes might bo 

 found in the sexual generation of higher cryptogams (which corresponds 

 with the cells derived from the pollen-grain, or from the mother-cell of 

 the embryo-sac). This surmise quickly became a certainty. Overton 

 himself discovered ('93) that the cells of the endosperm in the 

 Gymnosperm Ceratozamia divide with the reduced number, namely 

 eight ; and Dixon observed the same fact in Finns at the same time. 

 In the following year Strasburger brought the matter to a definite 

 conclusion in the case of a fern (Osmunda), showing that all the cells 

 of the protJiallium, from the original spore-mother-cell omvards to the 

 formation of the germ-cells, have one-Jialf the number of chromosomes 

 found in the asexual generation, namely twelve instead of twenty-four; 

 in other words, the reduction takes place in the formation of the spore 

 from which the sexual generation arises, scores of cell-generations 

 before the germ-cells are formed, indeed before the formation of the 

 body from which these cells arise. Similar facts were determined by 

 Farmer in Pallavicinia, one of the Hepaticae, where all of the nuclei 

 of the asexual generation (sporogonium) show four chromosomes dur- 

 ing division, those of the sexual generation (thallus) eight. It now 

 seems highly probable that this will be found a general rule. 



The striking point in these, as in vom Rath's and Hacker's obser- 

 vations, is that the numerical reduction takes place so long before 

 the fertilization for which it is the obvious preparation. Speculating 

 on the meaning of this remarkable fact, Strasburger advances the 

 hypothesis that the reduced number is the ancestral number inherited 

 from the ancestral type. The normal, i.e. somatic, number arose 

 through conjugation by which the chromosomes of two germ-cells 

 were brought together. Strasburger does not hesitate to apply the 

 same conception to animals, and suggests that the four cells arising by 

 the division of the oogonium (egg plus three polar-bodies) represent 

 the remains of a separate generation, now a mere remnant included 

 in the body in somewhat the same manner that the rudimentary pro- 

 thallium of angiosperms is included in the embryo-sac. This may 

 seem a highly improbable conclusion, but it must not be forgotten 

 that so able a zoologist as Whitman expressed a nearly related 

 thought, as long ago as 1878: "I interpret the formation of polar 

 globules as a relic of the primitive mode of asexual reproduction." 

 Could Strasburger's hypothesis be substantiated, it would place the 

 entire problem, not merely of maturation, but of sexuality itself, in 

 a new light. 



1 '78, p. 262. 



