MEIOTIC PHASE IN MARSUPIALS 197 



of the chromosome substance on the ground that they only 

 appear in tissues treated in a certain way, would be as illogical 

 (granting that it is true) as to deny the distinction between 

 a plasmosome and a chromatin nucleolus because the differ- 

 ence only becomes visible under the action of appropriate 

 stains. 



We conclude, therefore, that the chromomeres which appear 

 in certain stages of mitosis in fixed tissues correspond to real 

 local differentiations of the substance of the chromosome, 

 though the actual shape which they assume (namely, bead-hke 

 swellings on a fine thread) may be assumed, or at least exag- 

 gerated, under the stress of the fixative. 



(2) Crossing over. — Whether or not the phenomenon of 

 crossing over occurs in mammals is still in douJ^t. Castle 

 (1921) has described such a case in rabbits for the linked genes, 

 English and non-English, and short-haired and Angora. 

 Three individuals were tested, two males and a female, and 

 crossing over was found in all of them. If this is estabhshed, 

 it will show that the phenomenon in mammals is not quite 

 comparable to that in Drosophila and Bombyx, where 

 it occurs only in the sex which is homozygous for the sex 

 chromosomes. By analogy with these, crossing over is to be 

 expected in the female mammal alone. Considering the 

 cytological evidence only, it would certainly seem that the 

 conditions supposed to be necessary for crossing over are 

 provided in the male diplotene nuclei of both these genera. 



This is specially clear in Petauroides, because of the 

 chromomeres. As fig. 27 shows, fusion of chromomeres in 

 syndesis is intimate. Indeed, no sign of duplicity may remain. 

 In fig. 35 (diplotene stage) the intertwined chromosomes are 

 still held together at certain of the crossing places by unsplit 

 chromomeres, and in view of their intimate union it is not 

 difficult to imagine that when they finally do separate they may 

 do so in such a way that the portions of the two chromosomes 

 on either side of the point of union have been interchanged. 



(3) The Eelation between the Sex Chromosomes 

 and the Plasm osomes in the Meiotic Phase. — The 



