CHKOMOSOMES IN THE SPERMATOGENESIS OF THE HEMIPTERA HETEROPTERA. 145 



tliat the larger monosomes are bivalent elements. This idea of the bivalence of the 

 monosomes I carried out further in my last paper (1905). This seemed to me to best 

 explain the usually relatively large size of the monosomes. Since then McClung 

 (1905) has demonstrated the occurrence of undoubted bivalent chromosomes in the 

 spermatogonia of certain Orthoptera, which may be a union of two or more autosomes 

 or of a monosome with an autosonne. 



But Miss Stevens (1905?;) showed for Teiicbrlo that while in the spermatogenesis 

 there is a pair of diplosomes of very unequal volume, this pair is represented in the 

 ovogenesis by two of equal volume. Then Wilson (19066) compared the ovogenesis 

 and spermatogenesis in a series of Hemiptera, confirming Miss Stevens' conclusion and 

 elaborating it; Wilson's results may be l)riefly summarized as follows. Where there 

 is a single monosome in the spermatogenesis (as in Protenor, Harmostes, Anam. and 

 Alychis) there are two in the ovogenesis so that the ovogonia possess always an equal 

 number of chromosomes. And where in the spermatogenesis there is a pair of diplo- 

 somes of unequal volume, there is in the ovogenesis a pair with components equal in 

 volume to the larger diplosome of the spermatogenesis. Thus while half the sperma- 

 tids lack the monosome, and half of them lack the larger diplosome, each ovotid 

 would contain a monosome and each a larger diplosome. And from this phenomenon 

 Wilson concludes, as did Miss Stevens before him, that a spermatozoon containing a 

 monosome or the larger diplosome on fertilizing an egg produces a female individual ; 

 but that a spermatozochi lacking either of these gives rise to a male individual. 



The point in this important discovery of Wilson's that immediately concerns us 

 is that the modification of auto.somes into allosomes has taken place in the spermato- 

 genesis ; and that a monosome of the spermatogenesis has originated by the continuance 

 of the larger element of a diplosome pair in the sperm cells, and the loss of the smaller 

 element tliere. This is a very plausible conclusion, but there are in particular two phe- 

 nomena that must he explained before it can be accepted. One is, how an allosome 

 becomes lost in the spermatogenesis ; and the other is, how the allosomes introduced by 

 the spermatozoon into the ovum behave during the ovogenetic cycle; on both of these 

 questions we know as yet practically nothing. I showed in 1904 for Anam, that the pair 

 of minute diplosomes of the spermatogonium are represented in the ovogonium by a 

 pair equivalent in size and appearance. Such equivalent diplosomes we have just found 

 to be probably the least modified kind of allosomes. The commencement of the allo- 

 somes may have had then a parallel course in the two sexes. And the point that now 

 needs to be determined is the behavior of the ovogenetic allosomes in the "growth period 

 and the maturation divisions. 



So we have reached the conclusion that the allosomes are to be considered modi- 



A. p. S.— XXI. p. 24, 8, 'OG. 



