The Germ Cells of Aphis. 323 



The chromosomes retain their individuality in the spermatids 

 for some time, but finally become massed together to form the 

 sperm-head (Fig. 42). The development of the spermatozoon 

 from the spermatid appears to be very simple, the head being 

 formed mainly from the chromatin and the long, rather thick tail 

 from the cytoplasm. None of the accessory structures described 

 for other insects are present. No centrosome has been detected 

 in any mitosis, and asters have been seen only in connection with 

 the sperm-nucleus in the egg, the pronuclei, and the segmentation 

 spindles of the winter egg. There is no trace of anything that 

 could be called an "accessory chromosome" (McClung, '02). 

 The nucleolus appears to be of the same character throughout, 

 appearing in resting cells and disappearing in mitosis. That it is 

 not a chromatin nucleolus, or karyosome, is shown by the fact that 

 the chromosomes in many cases are visible before it disappears 

 (Fig. 29) and with the Delafield-orange combination the nucleolus 

 invariably takes the orange stain while the nuclear reticulum 

 stains with the haematoxylin. 



V. GENERAL DISCUSSION. 

 I. MendeVs Law, and the Individuality of the Chromosomes. 



It appears that in the Aphids studied there is a series of 5 chro- 

 mosomes of different shape and size in the germ cells of the sexual 

 generation: the maternal or egg-series is exactly equivalent to the 

 paternal or sperm-series. The chromosomes show the same rela- 

 tive form and size throughout the maturation divisions. The two 

 series of chromosomes meet in fertilization, and throughout the 

 parthenogenetic generations, both female and male, we find the 

 double series, 10 chromosomes of five different sizes. In the 

 spermatocytes, and presumably in the oocytes, the chromosomes 

 of the double series are paired, and in one of the maturation divi- 

 sions, apparently the first, the paired chromosomes are separated. 

 Supposing that the different chromosomes have different physio- 

 logical values, or represent different hereditary characters, as 

 maintained by Boveri ('02), we find in the behavior of the chro- 

 mosomes in the germ cells of the Aphid exactly the conditions re- 

 quired by Mendel's Law of Heredity. The characters represented 

 by the 5 constantly different chromosomes would be segregated at 



