664 PATTERNS AND PROBLEMS OF DEVELOPMENT 



during the development.'^ The possibility that the "most arterial" region 

 where the artery enters the follicle and begins to branch becomes dorsal 

 may be noted. A study of the development of circulation in relation to 

 pigment development in earlier stages is needed to clear up the question. 

 Meanwhile it appears probable that polarity of the frog egg is determined 

 by the circulation differential. 



FOLLICULAR DEVELOPMENT 



Both imbedded and pedunculate types of oocytes in many animals 

 are surrounded by a follicle, at least in early stages of growth. Cells ap- 

 parently nutritive in function aggregate about the oocytes of certain 

 sponges; epithelial cells may be stretched into a thin follicular membrane 

 which ruptures and disappears sooner or later as the oocyte grows (Fig. 

 213, A; also various echinoderms), or may persist until it is full grown 

 (holothurians). The temporary follicular membranes probably have little 

 or no special function in some forms but are merely cells so situated that 

 they are stretched into a membrane by growth of the oocyte. In some 

 groups, however — for example, cephalopods, some insects, and chordates 

 — the follicle becomes an epithelium of considerable thickness, consisting 

 of a single cell layer or, as in most chordates, several cells thick. Since it 

 surrounds the oocyte, this must obtain nutritive material from or through 

 it. The follicular epithelium of cephalopods develops numerous folds 

 which grow into the cytoplasm of the oocyte. From the ascidian follicle 

 the "test cells" pass into the oocyte. The follicle may also secrete a chori- 

 on. Since the more highly developed follicles usually surround the oocyte 

 uniformly, they apparently do not provide an environmental difierential. 

 The growing oocytes of chordates within their follicles usually approach 

 and protrude from the surface of the ovary, and in many forms the follicle 

 becomes more or less pedunculate. This course of development may sub- 

 ject the oocyte to a differential between ovary and body cavity. The 

 possibility that the follicular circulation may provide a differential de- 

 terminating polarity in the case of the frog egg was noted above. The 



'•< Bellamy has sometimes been cited as having abandoned, in the second paper (1921), the 

 view advanced earlier that polarity might be determined in relation to the circulation. He 

 did retract his earlier statement that "in every case observed the greater part of the arterial 

 blood supply was restricted to the pigmented hemisphere," because he found some cases in 

 which this was not true; but he emphasized the point that these concern only full-grown or 

 nearly full-grown oocytes and that they do not affect the earlier statement that the peduncle 

 is within 20° of the equator in 75-80 per cent of full-grown eggs with arterial circulation 

 largely over the pigmented hemisphere. 



