ORIGINS OF EMBRYONIC PATTERNS 671 



oocytes, as is generally agreed — it may be a matter of chance in some 

 species, perhaps in the bee (Fig. 218, E), which become oocytes, which 

 nutritive cells. However, in forms such as Forficula (Fig. 218, A), in 

 which the basal cell of a pair becomes the oocyte, it is evidently not chance 

 but a definite spatial relation of some sort that determines the difference. 

 Also, the definitely directed association of oocyte with a cell or a group 

 of cells apical to it is not a matter of chance. 



Recent observations on differential reduction in ovaries of Drosophila 

 hydei show a basipetal gradient of decreasing rate of reduction in the 

 ovaries as a whole and in each follicle string. The basal cell, that is, 

 the cell at the low end of this gradient in each follicle, becomes egg, 

 the others nutritive cells. The polar gradient of the egg is a part of the 

 ovarian and follicular gradient. A gradient from the ovarian surface in- 

 ward corresponds to the ventrodorsality of the egg, the ventral side 

 being at or toward the outer ovarian surface (see also p. 144; Child, 

 unpublished). 



EGG POLARITY : GENERAL 



Eggs of different species undergo their pre-embryonic development 

 under widely different conditions. The polar patterns of many are obvi- 

 ously related to differential conditions in their environment, but our 

 knowledge of environmental conditions affecting the developing egg cell 

 in the parent organism is a matter of inference from observation rather 

 than of experimental demonstration. In some forms there is apparently 

 a differential between free and attached pole, such that the free pole be- 

 comes apical, perhaps because respiratory exchange is more rapid there, 

 while nutritive material enters at the attached pole. In other forms the 

 pole through which nutritive material enters becomes apical, perhaps be- 

 cause of a gradient in the ovarian tubule of which the oocyte is a part, 

 as in insects, perhaps because a circulatory system is the chief mediator 

 of respiratory exchange as well as source of nutritive material. In still 

 other forms there is evidently relation to a factor external to the egg cell, 

 archegonium, gametophyte, and embryo sac in plants, nutritive cells, 

 follicles, etc., in animals; but we have even less basis for a guess as to 

 the nature of the effective factor. And finally, in some forms the oocyte 

 is isolated in a fluid medium during its development from a primitive 

 germ cell. Developmental stages of most egg cells are so inaccessible 

 to present experimental methods, and interest has been so largely cen- 

 tered in the egg cell itself, that physiological investigation of the origins 

 of egg polarity is almost completely lacking. 



