1905.] NATURAL SCIENCES OF PHILADELPHIA. 201 



then in the course of time all the chromosomes would become chromatin 

 nucleoli; yet in no individuals were found more than one pair. So from 

 whatever standpoint we regard his explanation its improbability be- 

 comes manifest. On the other hand his chromatin nucleoli behave 

 exactly like the chromatin nucleoli (paired heterochromosomes) of 

 Euchistus, except that they are not distinguishable in the spermato- 

 gonia (in some other Hemiptera they are also not recognizable in these 

 cells) ; so there is every reason to consider them as persisting from indi- 

 vidual to individual as chromatin nucleoli. What he calls in Syro- 

 mastes the accessory chromosomes are not heterochromosomes at all, 

 so certainly not later stages at all of chromatin nucleoli, for he describes 

 them as conducting themselves exactly like the ordinary chromosomes 

 during the growth period ; the bivalent accessory chromosome of the 

 spermatocytes differs only from the other bivalent chromosomes in 

 failing to divide in the second mitosis. I think this " accessory chromo- 

 some " of Syromastes is to be considered a stage leading to that of the 

 unpaired heterochromosome of Protenor; they resemble each other in 

 failing to divide in the second maturation mitosis, and though the one 

 in Protenor is virtually single in the spermatogonia we have given 

 reasons to show that it is probably bivalent there. The failure to 

 divide in the second mitosis can for both be ascribed to incompleteness 

 of the longitudinal split. And this is surely a far simpler interpretation 

 of the phenomena in Syromastes, one much more in accordance with 

 what has been described in other objects, than that elaborated by 

 Gross. It is hardly necessary to adjoin that such a process as the 

 two maturation rlivisions of one pair of chromosomes being continued 

 over two germinal cycles has no known counterpart in other animals, 

 and so needs the most rigid observational demonstration, 



Gerard (1901) has described for Prosthecerceus and Schockaert (1901) 

 for Thysanozoon a peculiar deep-staining thread within the ovocytes 

 which divides into two, and is said to give rise to the egg centrosomes ; 

 it is for future research to determine whether this structure may have 

 any relation to the heterochromosomes. 



