200 PROCEEDINGS OF THE ACADEMY OF [Feb., 



ideas of Gross (1904). He describes for Syromastes, a Hemipteron, two 

 pairs of modified chromosomes: one pair of these, which he calls chro- 

 matin nucleoli, differ from the other chromosomes in acting like hetero- 

 chromosomes during the growth period of the spermatocytes, but agree 

 with them in dividing in both maturation mitoses; the other pair, which 

 he calls accessory chromosomes, differ from the ordinary chromosomes 

 in not dividing during the second maturation mitosis, but behave ex- 

 actly like them during the growth period. His chromatin nucleoli, which 

 are not recognizable until the stage of the spermatocytes, are said not 

 to differ in volume from the ordinary chromosomes in the spermatogo- 

 nia; while his accessory chromosomes are described as the smallest of 

 all the chromatin elements. Both kinds of these bodies are paired and 

 univalent in the spermatogonia, and by conjugation become bivalent 

 in the spermatocytes. Now Gross reasons these are separate genealogi- 

 cal conditions of one and the same structure. He argues that a pair 

 of unmodified ordinary chromosomes of the spermatogonium become 

 in the spermatocytes chromatin nucleoli, which there act like hetero- 

 chromosomes, preserve their compact structure and undergo no longi- 

 tudinal split, and divide in both maturation mitoses, so that each sper- 

 matid receives a half of each univalent component. A spermatozoon 

 formed from such a spermatid unites with an ovotid with a correspond- 

 ing semivalent chromatin nucleolus. But instead of these two semi- 

 valent heterochromosomes (chromatin nucleoli) of the fertilized egg 

 appearing in the next following generation of spermatogonia as chro- 

 matin nucleoli, he conceives them to appear in the form of the pair of 

 small accessory chromosomes, which form a bivalent one in the follow- 

 ing spermatocyte, divide in the first maturation mitosis but not in the 

 second, so that half of the spermatids receive a half of each of them. 

 So he interprets them both as chromosomal elements whose maturation 

 divisions are continued over two generations of individuals; although 

 he really describes three divisions of them, two for the chromatin nucle- 

 oli and one for the accessory chromosomes. We need not enter here 

 upon his further deductions from this interpretation, but shall consider 

 simply its probability. A strong objection that suggests itself is this : 

 all the individuals studied by him showed in the spermatocytes two 

 chromatin nucleoli and two accessory chromosomes; but this would 

 be impossible if in every other generation the chromatin nucleoli 

 changed into accessory chromosomes, for then one should find in the cells 

 of some individuals no chromatin nucleoli but four accessory chromo- 

 somes. And if, and Gross suggests this possibility, from time to time 

 successive pairs of ordinary chromosomes become chromatin nucleoli. 



