OF TIIK GE&M CELLS OF METAZOA. 



215 



five (or six) chromosomes conjugates with an ovum with five (or six), so that each of the 

 conjoints has the same number, an even number would result in the fertilized ovum. But 

 if a spermatozoon with five (or six) chromosomes unite with an ovum with six (or five), 

 the conjoints having then different numbers of chromosomes, the fertilized ovum would 

 have the uneven number eleven ; the uneven number would then be perpetuated from 

 individual to individual, so long as the conjugating cells have different numbers of chro- 

 mosomes, and so long as the odd chromosome does not divide in one of the maturation 

 mitoses. (2) Or germ cells from individuals with an uneven normal number of chromo- 

 somes, by conjugating with germ cells from individuals with an even number, would occa- 

 sion an uneven number in the fertilized ovum. Either of these possibilities would suffice 

 to explain the transference of the uneven number from individual to individual, though 

 the first possibility would appear the more probable. 



Ho far we have considered the origin of the uneven chromosomal number and the 

 mode by which it is perpetuated from individual to individual. We have now to discuss 

 its significance. Most of the Hemiptera examined by me show an even normal number of 

 chromosomes; only four showed an uneven number, and in no other Metazoa has an un- 

 even number, to my knowledge, been found. The uneven number would accordingly 

 appear to be unusual. It seems to me probable that the uneven number represents a 

 transition stage between a higher number and a lower, or the converse, and it is unusual 

 because the transition stage is probably shorter than the earlier and the later stages. The 

 number of the chromosomes varies quite considerably in the different species of the Hemip- 

 tera heteroptera, but we cannot suppose that the number was constant for each species 

 from the beginning any more than we can consider that the species have always remained 

 unchanged; there must have been an evolution of the chromosomal number, as there has 

 been of the species. It is quite possible that an even number of chromosomes, as e.g. 

 twelve, may have changed into an even number (ten) without first passing through the stage 

 of the uneven number (eleven). Tins might take place by the number ten appearing 

 simultaneously in both paternal and maternal germ cells through some abnormality or 

 deficiency in mitosis. But it is far more probable that such a mitotic abnormality would 

 not occur coincidently in both kinds of cells — more probable, e.g., that a paternal germ 

 cell, acquiring an abnormal number of chromosomes by some fault in the process of mitosis, 

 would conjugate with a maternal germ cell with the normal number; the result of such 

 a, union would be of course an uneven number. On this argument, when the chromo- 

 somal number changes, the period of change would be characterized by an uneven num- 

 ber of chromosomes. Ultimately an even number of next lower or next higher order 

 would be reached, and that number must persist longer than the uneven number in view 

 of the fact that uneven numbers are comparative rarities. If both paternal and maternal 



A. F. S. — VOL. XX. B.B. 



