ORIGINS OF EMBRYONIC PATTERNS 665 



follicular epithelium surrounding each oocyte in insect ovarian tubules 

 is probably a part of a longitudinal gradient in the whole tubule. 



DEVELOPMENT WITH ACCESSORY OR NUTRITIVE CELLS 



Oocytes of many animals grow at the expense of other cells, the so- 

 called "accessory" or "nutritive" cells. These may be more or less com- 

 pletely resorbed during growth of the oocyte. Very commonly the ac- 

 cessory cells are from the same region of the ovary as the oocyte and 

 are usually believed to possess the same potentialities as the cell that 

 becomes an egg. If this is the case, some factor in the environment or 

 relations to each other of the cells must determine their respective fates 

 as oocytes or accessory cells. Nutritive cells vary from one to a consider- 

 able number for each oocyte, but number and position in relation to the 

 oocyte are usually more or less definite for the species. 



Association of oocytes and accessory cells is frequent among polychete 

 annelids. The oocyte of Ophryotrocha is accompanied by a single cell, 

 larger than the oocyte in early stages (Fig. 216, A, B)y= The paired cells 

 separate from the ovary and become free in the body cavity at an early 

 stage. Braem concludes that cells exposed to body fluid at the surface of 

 the ovary become accessory cells, each serving as a source of nutrition to 

 the cell immediately beneath it; the two cells become attached to each 

 other, the one originally below the ovarian surface becoming oocyte. In 

 the single observed case of polar-body formation with attached nutritive 

 cell, this cell was at the basal pole (Braem, 1894). 



The primitive germ cells of the polychete Diopatra migrate from the 

 primary into a secondary ovary; there they divide to form chains, con- 

 sisting of a variable number of cells. In general, cell size decreases from 

 the middle toward both ends of the chain; one cell of the middle region 

 becomes larger than the others, protrudes from one side of the chain, and 

 develops as oocyte, all other cells of the chain being nutritive (Fig. 216, 

 C, D). As the oocyte grows with progressive ingestion of nutritive cells, 

 it protrudes increasingly from one side of the chain, so that finally remain- 

 ing nutritive cells of both terminal portions of the chain come to lie to- 

 gether at one pole (Fig. 216, E)\ this becomes the basal pole (Lieber, 

 193 1). Apparently there is some degree of physiological integration in 

 the chain; the gradation in cell size from the middle suggests a differential 

 of some sort in both directions as a possible factor determining which cell 



'5 Braem, 1894; Korschelt, 1894. 



