ORIGINS OF EMBRYONIC PATTERNS 



663 



In general, the peduncle probably plays a part in the transport of 

 nutritive material to the cell. The oocyte often shows a special structure 

 radiating from it (Fig. 213, A), but among invertebrates the nucleus often 

 lies nearer the free pole. Respiratory exchange and elimination of prod- 

 ucts may be more rapid there. In forms with a well-developed circulatory 

 system and with oxygen supplied chiefly by the blood respiratory ex- 

 change may perhaps be greater at the attached pole. At any rate, condi- 

 tions are doubtless more or less different at free and attached poles; that 

 these differences may deter- 

 mine egg polarity in many cases 

 is indicated by the evidence. 



The oocyte of the frog is sus- 

 pended from the ovarian wall in 

 an epithelial sac, the theca; be- 

 neath that is a thin follicular 

 membrane. The theca forms a 

 short peduncle, an artery and 

 vein develop in it, and a much 

 branched capillary circulation 

 surrounds completely the de- 

 veloping egg cell. In 75-80 per 

 cent of full-grown oocytes taken 

 at random from different ovari- 

 an regions the boundary be- 

 tween pigmented and unpig- 

 mented zones is within 20° of the peduncle. Injection of the vessels with 

 colored masses and direct observation of movement of corpuscles shows 

 that usually the greater part of the arterial circulation is over the pigmented 

 hemisphere (Fig. 215). There is, however, considerable variation, and 

 cases have been found in which the arterial circulation is largely over the 

 unpigmented hemisphere (Bellamy, 1919, 192 1). Nevertheless, the high 

 frequency of close correspondence between arterial circulation and the 

 region of the oocyte in which pigment develops, the apical or animal 

 hemisphere, is far above probability. Moreover, it is probable that full- 

 grown oocytes may change position in the theca in consequence of me- 

 chanical conditions resulting from the ovarian contractions or other 

 movements of the animal, for they are not in any way attached to it. 

 Bellamy has also suggested the possibility that the circulation may change 



Fig. 215. — Full-grown ovarian oocyte of frog 

 {Ranapipiens), showing the usual relation between 

 arterial and venous circulation in the theca and 

 pigmented and unpigmented regions (from Bellamy, 

 1919). 



