380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



aj-cliromosomes, the others collectively beinjj; known as the aiito- 

 chroniosomes, or orthochromof^oiiws. One a;-chromo.sonie came from 

 the female parent cell, the other from the male. At synapsis the 

 two .^'-chromosomes pair with each other, prodiicint^ one double 

 a'-chromosome. Thus, for example, if an egg cell contained orig- 

 inally six chromosomes in all (fig. 4 A), two of which were 

 a?-chromosomes (a?, a?), its chromosome formula after synapsis is 

 2-[-lft? (fig. 4 C), hut W'itli each chromosome now of double value. 

 With all egg cells having an even number of cliromosomes, as we 

 have seen most female cells have, the mating of tlic clnoinosomes in 

 synapsis reduces the number of chromosomes to just one-half tlie orig- 

 inal number. Chromosome reduction in the egg, tlierefore, is a simple 

 matter of dividing by two in most cases. But what happens during 

 synapsis to a male germ cell with an odd number of chromosomes? 

 Simply the most natural thing: The corresponding orthochromo- 

 somes pair with each other; the .7?-chromosome, being without a 

 mate, remains single (fig. 4 D, E, F). Hence, in a male cell with 

 five original chromosomes, the chromosome formula after synapsis 

 becomes 2-j-la?, the same as in the egg cell of the same species, except 

 that the .r-chromosome is of single value. In male cells that have 

 an even number of chromosomes, the a-chromosome has a small 

 mate, which is designated the y-chromosome. After synapsis in 

 one of these cells having ^-\-x-\-y chromosomes, the chromosome 

 formula becomes 2-|-a?y, in which xy represents a single double 

 chromosome. 



After the chromosomes are paired off, the chromatin material 

 may again become strung out in the form of threads, but in any 

 case the threads soon take on the typical chromosome forms as in 

 the stage preceding cell divisions, for there is now about to take 

 place two special divisions of the germ cell which wall prepare 

 it for the eventual union with a germ cell of the opposite sex in 

 fertilization. These two divisions of the germ cells constitute the 

 process known as maturation. Synapsis, in nearly all cases in 

 animals, precedes the first maturation division; in a few species of 

 insects, however, it precedes the second division. 



The insect egg up to the time of maturation has been still within 

 one of the egg tubes of the parent female, wdiere it has grown 

 enormously by the accumulation of yolk in its j^rotoplasm. In its 

 early stages it divided entire, like any other cell, but now^, on ac- 

 count of its bulk, it can no longer conveniently do so. Hence- 

 forth all its divisions will affect only the nucleus. At this stage the 

 egg is usually laid, and maturation takes place immediately after- 

 ward. At the beginning of maturation the nucleus moves nearer 

 the surface, and at the first maturation division the halves of each 



