Il6 N. M. Stevens 



difference being that one-half of the second spermatocytes degener- 

 ate, leaving only one kind of spermatozoa — those destined to 

 unite with female-producing eggs. 



These facts, which are described as quite clear for Phylloxera, are 

 exceedingly obscure for the aphids, and not until I had gone over 

 all of my material three times, was I able to convince myself that 

 the lagging chromosome does not divide in the first spermatocyte, 

 but in the telophase goes into the larger of two second spermato- 

 cytes, the smaller cell sooner or later degenerating. The earlier 

 stages shown in previous figures indicate equal division of chromo- 

 somes and cytoplasm; and in the final division of the cell there is 

 every possible variation of inequality. There is also great varia- 

 tion in different species in the smaller second spermatocytes. In 

 some cases the chromosomes massed in the anaphase of the first 

 spermatocyte, never separate again, and one finds these rounded 

 clumps of chromatin with little cytoplasm among the dividing 

 second spermatocytes, while in the youngest cysts of spermatids 

 they have completely disappeared. In other cases the chromo- 

 somes separate and show no signs of degeneration in the second 

 spermatocyte cysts, and they may even divide, mitosis going as 

 far as a late anaphase, but not ending in cell division. In the 

 latter case degeneration occurs in the younger cysts of spermatids. 



A few of the most convincing cases will now be described with 

 figures, the different aphids being designated by their host plant 

 as in previous papers. 



In the green rose aphid the anaphases (Figs, i, 2 and 3) would 

 not even suggest the possibihty that the lagging chromosome is 

 not equally divided between the two second spermatocytes, yet in 

 the prophase of the second division (Fig. 4) there is a double 

 chromosome which can be no other than the lagging one of the 

 first division with the two parts folded together. Fig. 5 is a 

 smaller second spermatocyte containing six chromosomes and 

 lacking the double one. Figs. 6 and 7 are first and second sper- 

 matocyte equatorial plates, and Fig. 8 is a side view of a first 

 spermatocyte spindle, showing the heterochromosome {x) not 

 yet divided, while the other chromosomes are in early anaphase. 



In the star cucumber aphid there is great variety in the ana- 



