288 GEORGE W. BARTELMEZ 



long axis. This eccentric position of the latebra plays an impor- 

 tant part in the orientation of the ovum in the oviduct as will 

 ap])ear below (p. 292). 



4. The zone radiata and the rotation of the oocyte. One of the 

 reasons whj^ the polarity of the bird's egg has not been understood 

 hitherto, is that in the period now under discussion all possible 

 relations are to be found between the follicle with its long axis 

 and stigma, and the polar axis of the oocyte. In the preceding 

 and the subsequent stages the relations are constant. Thus in 

 the former practically every follicle shows the polar axis perpen- 

 dicular to the long axis and many have the animal pole coinciding 

 with the attached pole (diagram II, p. 279). During the final 

 growth period also, the conditions shown in diagram 2, E are 

 found almost invariably; i.e., the long "axis of the oocyte is identi- 

 cal with the stigmal axis of the folUcle and both are perpendicular 

 to the polar axis, the animal pole being at the attached pole of 

 the follicle. How does this come about? It might be supposed 

 that only those folUcles in which the 'typical' conditions exist, 

 enter upon the final growth period, but this is not so; for the 

 final growth of the oocyte is not dependant upon the relation 

 of the oocyte to its follicle. The facts are these: Oocytes less 

 than a millimeter in diameter usually show the 'typical' relations. 

 After the germinal vesicle has begun to migrate peripherally all 

 possible relations may be found between the oocyte and its fol- 

 licle, (fig. 7 shows an extreme case). During the final growth 

 period the 'typical' relations are again found, this time almost 

 invariably. The explanation was first suggested by some experi- 

 ments which showed that, in the later stages, the oocyte as a 

 whole is free to rotate within its folhcle. A bird with growing 

 oocytes was tied down on its back for twenty-four hours; when 

 the abdominal cavity was opened it was found that the germinal 

 vesicles occupied the highest points in the folhcles viz: the free 

 poles, and yet the structure of the eggs was normal. The polar 

 axis is therefore not influenced by gravity, as some authors have 

 implied; the oocyte as a whole simply orients itself with reference 

 to gravity. In applying these data to the stages in question the 

 following facts must be borne in mind: The oocytes are lying in 



