On Morphological Difference of the Chromosomes of Ascaris megaloceph. 67 
and by Häcker (1907) in bis recent admirable review, and to the 
papers therein cited are to be added those of Borixg (1907) and 
Stevexs (1908). Bnt wbile such constant size and form ditferences 
are most clearly and unmistakably marked on the allosomes they 
have also beeu observed on certain of the unmodified chromosomes 
of various species. Thus there have been made known a fairly large 
uumber of examples where in germ cells with the unreduced or nor- 
mal number of chromosomes, that is, in oögonia and spermatogonia, 
constant morphological ditferences of the chromosomes have been 
demonstrated, sometimes only for a single element (monosome), some- 
times for a single pair (then usually diplosomes), sometimes for two 
or three pairs, in a few cases constant size and form ditferences of 
all the chromosome pairs (Plethodon, certain Hemiptera and Diptera). 
Such clear cases constitute one of the strongest proofs of persisting 
chromosome individuality, and they have not received fair considera- 
tion at the hands of Fick (1905) and others who have sought to 
overthrow this idea of persisting individuality. 
Further, when in spermatogonia the chromosomes exhibit such 
constant morphological ditference, the number of kinds of chromo- 
somes is determined to be one half the total number of chromosomes 
(when the latter number is an even one), that is to say, pairs of like 
chromosomes can be distinguished, the two elements of each pair 
being more like one another than they are like the elements of any 
other pair. But in the reduced number of chromosomes, such as is 
found in the spermatids, no such pairs can be defined but all the 
chromosomes are found to be morphologically ditferent. This coin- 
cidence is comprehensible only on the assumption, for which there 
is much observational evidence, that like chromosomes, i. e., the Hvo 
of any pair, conjngate in the first spermatocytes and are later sepa- 
rated by a reduction division (Moxtgomery, 1901, 1906). This con- 
sideration has been left entiredy out of account by those who still 
argue against the occurrence of a reduction division, namely Boxxe- 
viE (1906) and Vejdovsky (1907), and no explanation is given for 
it when it is assumed that both inaturation divisioiis are equational. 
The only contribution so far made to examine these phenomena 
in the Ascaris egg were presented in a short paper (1904) by me, 
in which the conclusion was reached, from a study of the chromo- 
somes of the polar spindles as well as of the first cleavage, that 
the egg furnishes one larger and one smaller chromosome, that the 
sperm cell introduces one larger and one smaller, and that in the 
