304 T. H. MORGAN 



THE OMISSION OF SYNAPSIS IN THE PARTHENOGENETIC EGGS 

 OF PHYLLOXERANS AND APHIDS 



As is well known, the full or diploid number of chromosomes is 

 present in most eggs that develop by means of parthenogenesis. 

 Whether the presence of the full number of chromosomes has in 

 itself anything to do with the phenomenon of parthenogenesis 

 may well be disputed, because in some forms, as in the male bee, 

 the eggs develop without being fertilized with half the number of 

 chromosomes and in artificial parthenogenesis the half number of 

 chromosomes occurs in some forms, at least. In the phjdlox- 

 erans and aphids there is a loss of one or two chromosomes from 

 the male-producing egg that develops by parthenogenesis. 



There is a further question that is important from a descriptive 

 cytological point of view, namely, whether the parthenogenetic 

 eggs omit the synapsis stage and retain in consequence the full 

 number of chromosomes, or whether they pass through such a 

 stage and the chromosomes subsequently separate. In phjdlox- 

 erans and aphids the case is quite clear^ and I wish to emphasize 

 the ease and certainty with which the problem can be studied 

 in them. 



In the bearberry aphid, the ovary that is producing sexual 

 eggs (diagram 3) can with certainty be distinguished from the 

 ovary that is going to produce parthenogenetic eggs (diagram 2) . 

 In the latter there is no contraction phase of the chromosomes. 

 A prophase of an oogonial division of a parthenogenetic egg is 

 shown in diagram 2, a. Six chromosomes are distinctly seen and 

 the same number is found in the equatorial plate stage shown in 

 diagram 2, &. At the beginning of the growth period, when the 

 chromosomes begin to take the stain again, scattered threads or 

 strands can be made out, as shown in c, which by further con- 

 traction, d, give rise finally to the six rod-like chromosomes. 

 In later stages, when the egg is about ready to leave the ovary 

 and after that time while it is still acquiring yolk, the six chromo- 

 somes can be distinctly seen and easily counted, as shown by most 

 of the eggs in diagram 2, e and/. In the ovary of the sexual in- 

 dividual the chromatin begins to condense into threads at the 



' Morgan, T. H. Proc. Soc. Exp. Biol, and Med., vol. 7, 1910. 



