ORIGINS OF EMBRYONIC PATTERNS 66i 



comes the apical pole of the embryo, according to Boveri (1901a, b); 

 the free pole is apical, according to Jenkinson (191 16). Recently the find- 

 ing of maturation spindles at the free poles in two species seems to have 

 settled this question in favor of the free pole as apical (Tennent, 1931; 

 Lindahl, 19326). There was a similar difference of opinion concerning 

 holothurians, but here also it has been found that the free pole becomes 



apical/" 



The nucleus of the asteroid oocyte usually comes to lie close to the 

 surface of the cell at some region nearer the free than the attached pole, 

 but without other definite relation to the free pole." Polar bodies form 

 in Asterias in the region where the nucleus is nearest the cell surface 



Fig. 214.— Positions of polar body in attached eggs of the starfish Paliria miniata (from 

 Child, 1936a). 



(Wilson and Mathews, 1895). According to Yatsu (1910a), eggs in which 

 free and attached poles can be distinguished by shape of the egg and 

 presence of follicular membrane invariably form polar bodies halfway be- 

 tween the equator and the rounded free pole. Polar-body formation in 

 Patina eggs still attached shows no constant relation to attached or free 

 pole but depends entirely on position of the nucleus, and this appears to 

 be determined by chance conditions, except that it is commonly nearer 

 the free than the attached pole and may sometimes be at the free pole 

 (Fig. 214). The diiJerential between attached and free pole may be less 

 in the starfish than in the sea urchin, or the nucleus may be less sensitive 

 to it, or it may be that the nucleus comes to lie near the surface where the 

 oocytes are less closely packed together and there is more circulation of 

 fluid. On the other hand, it is possible that the nucleus lies where it 



" Gerould, 1896; Theel, 1901; Oshima, 1921; Inaba, 1930. 

 ^' Asterias forbesii, Paliria miniata, author's observations. 



