186 CYTOLOGY CHAP. 



In the following three cases the meiotic disturbance must be ascribed 

 to physiological causes, since the degenerative changes that take place 

 in the nucleus are more profound, even though in some cases the number 

 of chromosomes in the parent species is the same. 



Matings between the magpie pigeon ( <$ ) and dove ( ? ) (Smith, 1913) 

 result in male offspring only. These are found to exhibit a meiosis 

 differing from the normal in that there is an irregular metaphase I. 

 and, doubtless in consequence of this, the second meiotic division appears 

 to be omitted. In any case it was never found, though spermatogenesis 

 proceeds without it, resulting in the formation of spermatozoa, 77 per cent 

 of which are about twice the size. of those of either parent. The 



* , .?;; 



A 4 ^ *J * 



' ..** ** B " "C D '" 



FIG. 80. 



The chromosomes ol certain Lepidoptera and their hybrids. (After Harrison and Doncaster, J.G., 1914.) 

 All are polar views of equatorial plates. A, B, Lycia hirtaria. A, oogonium showing 28 chromosomes ; B, 

 spermatocyte I., 13 bivalents (one compound). C, D, It hysia zonaria. C, spermatogonium, about 112 chromo- 

 somes D, spermatocyte I., 56 bivalents. E, F, the hybrid, /. zonaria x L. hirtaria. E, spermatogonium, 14 large 

 (hirtaria), and about 56 small (zonaria) chromosomes ; F, spermatocyte I., 63 chromosomes. G, spermato- 

 cyte I. of the reciprocal hybrid, about 60 chromosomes, showing the two parental types. 



experiments indicate that the spermatozoa are not functional, for two 

 of the hybrid males were paired with female pigeons, which laid and 

 incubated eggs which, however, proved unfertile. 



In the hybrids between different species of pheasant (Smith and 

 Thomas, 1913), and pheasant and domestic fowl (Cutler, 1918), no normal 

 stages can be observed after synizesis. These hybrids are of course 

 sterile. When they are females the resulting anatomical abnormality 

 of the ovaries is very marked, since oogenesis ceases at synizesis and 

 therefore there is an entire lack of yolked oocytes. Thus the ovaries 

 remain minute and sometimes invisible. 



The best known sterile hybrid is of course the mule. The male 

 meiotic phase of this animal has been worked out by Wodsedalek (1916), 



