ORIGINS OF EMBRYONIC PATTERNS 667 



cells, as if each, or the region of the chain in which it formed, were to some 

 degree dominant over a certain length of chain. Somewhat similar cell 

 chains develop in ovaries of certain parasitic copepods."^ In some of these 

 the oocyte develops at one end of the chain; and when it separates, the 

 next cell becomes an oocyte, etc. In others the oocyte develops elsewhere 

 in the chain, but whether polarity is determined in relation to the acces- 

 sory cells seems not to be known. 



Two opposite regions of the Myzostoma oocyte, each associated with a 

 single nurse cell which is completely incorporated at an early stage, are 

 believed to become respectively apical and basal poles (Wheeler, 1896, 

 1897). Whether position of the accessory cells results from a pre-existing 

 polarity of the oocyte or oogonium or determines polarity is not clear. 

 If the nurse cells are alike, as they appear to be, their association with the 

 two poles of a predetermined heteropolar pattern is puzzling; on the other 

 hand, how they can determine a heteropolar axis if they are alike is not 

 evident. If they differ from each other, the question of the origin of these 

 differences arises. Moreover, they are sometimes together, and occasion- 

 ally there is only one. 



In another annelid, Tomopteris elegans, groups of eight cells each, at 

 first all of the same size and similar in appearance, occur in the ovary 

 and later become free in the body cavity. One cell increases in size and 

 becomes the oocyte; seven remain accessory cells (Fig. 216, F) but are 

 apparently not incorporated into the oocyte. Presumably each group de- 

 velops from a single ovarian cell; but, in any case, development of one 

 cell as the oocyte must be determined by a pattern of some sort in the 

 group with dominance of this cell. The origin and nature of this pattern 

 and the relation of egg polarity to it remain to be determined. 



In the oogenesis of the gephyrean Bonellia thin follicles are formed 

 about groups of cells at the surface of the ovary, become pedunculate, the 

 inclosed cells increase to a considerable number, a single cell at the proxi- 

 mal end of the follicle nearest the peduncle becomes oocyte, and all others 

 form a group at its distal pole. Since all cells of a follicle are apparently 

 alike in early stages, a differential must be present in the follicle to de- 

 termine the proximal cell as oocyte. With the follicular peduncle at one 

 pole and the group of accessory cells at the other the oocyte is probably 

 exposed to a differential adequate to determine its polarity. 



Association of oocytes with nutritive cells is very common among ar- 

 thropods. In the genesis of the parthenogenetic egg of the daphnid Sida 



^^ See, e.g., van Beneden, 1870; Kerschncr, 1879; Giesbrecht, 1882. 



