444 ROBERT T. HAXCE. 



optic cup, auditory vesicle, connective tissue, heart muscle, 

 blood, amnion, gonads and in tissue cultures of muscles. 



The Chromosome Number. The difficulties involved in de- 

 termining the exact number of chromosomes in the chick have 

 been described in detail (5). Briefly, the trouble encountered 

 may be said to be due, first to the extreme smallness of the 

 shortest chromosomes of the complex making observation at 

 times uncertain and second, to the apparent failure of the 

 component granules of the smaller chromosomes to unite or at 

 least to clearly indicate their proper relations to each other until 

 late metaphase if indeed it really and always happens then. 



The average number of chromosomes in the soma of the chick, 

 based on 78 counts, is 33. I think that this is perhaps lower 

 than the actual number which the most satisfactory counts 

 indicate to be about 35 or 36. This variation in the chick is 

 apparently due to a failure of the chromomeres or parts of the 

 smaller chromosomes to unite rather than to fragmentation as in 

 the case of the pig (i). This opinion is based upon the ob- 

 servations in the chick of larger numbers of distinct chromatin 

 bodies (with occasional visible connecting threads between them) 

 in the prophase than could be found in cells in later stages of 

 mitosis whereas in the pig a reduction in the chromosome number 

 as the metaphase was neared did not occur. The smaller 

 chromosomes of the chick are the only ones concerned while in 

 the pig the long ones are the ones that become broken up. 



The Chromosome Form. All of the longer chromosomes (about 

 1.2 in number) of the somatic cells are in the form of J's while 

 the shorter ones are rods. Their structure and size relations (5) 

 are alike in all the tissues studied including tissue cultures (4). 

 The largest chromosome (chromosome pair in the male) of tin- 

 complex has been shown to be the one associated with sex and is 

 found in all somatic cells as clearly as in the cells of the gonads 

 (5)1 F'gs. i to 9. The female embryonic cells are heterozygous 

 for this chromosome while the male cells are homozygous. No 

 differences in the chromosomes or their behavior have been 

 noted between any of the body tissues or in comparison with 

 those of the embryonic germ cells. 



