ORIGINS OF EMBRYONIC PATTERNS 



659 



gan, is not present. In many, if not in all, of these forms the oocyte is 

 subjected sooner or later in its development to an environmental differen- 

 tial. One pole of the ectodermal hydrozoan oocyte comes to be separated 

 from the water only by a very thin membrane and often protrudes from 

 the body surface as growth progresses (e.g., Hydra), while the other pole 

 is deeply imbedded and close to the entoderm, presumably the chief 

 source of nutrition. Usually the nucleus comes to lie near the outer pole 

 (Fig. 212, A, B). In the medusa Phialidium, in which the growing oocytes 

 form a columnar epithelium along the radial canals (Fig. 212, 5) and are 



A C D 



Fig. 212, A-D. — Hydrozoan oocytes. A, oocyte of Corymorpha in manubrium of medusa 

 bud with nucleus close to outer pole; 5, Phialidium oocytes, forming columnar epithelium; C, 

 D, gradient of Phialidium oocyte (from Child, 1925(7). 



attached by their inner poles, it has been shown that a gradient, indicated 

 by susceptibility and by reduction, is present in the advanced oocyte 

 (Fig. 212, C, D) and that the outer pole becomes the apical pole of the 

 embryo (Child, 1925a). The data suggest a gradient determined by greater 

 respiratory exchange at the outer, and uptake of nutrition at the inner, 

 pole. There may be two opposed overlapping gradients in concentration 

 of different substances, but the data available indicate one activity gra- 

 dient decreasing basipetally. Entodermal oocytes of some scyphozoa and 

 anthozoa attain somewhat similar relations to the coelenteric cavity. In 

 certain scyphozoa the entodermal epithelium in contact with a region of 

 the growing oocyte becomes thickened, forming a "cell crown" regarded 

 as nutritive in function, and the oocyte nucleus is at this pole (0. und R. 



