ORIGINS OF EMBRYONIC PATTERNS 689 



this pattern is predetermined in the oocyte independently of ovarian en- 

 vironment, as Bartelmez seems to believe, or is related to some environ- 

 mental factor, as seems possible, it is not evident that it has any neces- 

 sary relation to bilaterality of the embryo, for the median plane of the 

 embryo forms an angle varying from 8° to 135° from the median plane of 

 the oocyte, with a mode at 70°. Bartelmez also finds that in some oocytes 

 the shorter diameter of the nucleus is inclined to the long axis of the 

 oocyte by about the same angle as the embryonic axis to the long axis 

 of the egg, and he suggests that the embryonic axis may be determined by 

 the nucleus. He regards the polar axis as predetermined in the oocyte, 

 and polarity and symmetry as inherent properties of protoplasm, per- 

 sisting from generation to generation. If this were the case, however, it 

 would seem that much greater constancy of angle between embryonic 

 axis and long axis of the egg might be expected, but the relation between 

 the two axes gives a probability curve with less variation in eggs from one 

 bird than in those from more than one. In short, the data concerning 

 chick, pigeon, and other birds do not exclude the possibility that embry- 

 onic axiate pattern is not predetermined but originates in reaction to 

 some more or less variable factor in the organismic environment of the 

 oocyte or egg. The possibility that its determination may be correlated 

 in some way with position of the ovary on the left side of the body seems 

 not to have been considered. 



In mammals follicle development perhaps subjects the growing oocyte 

 to a definitely directed environmental differential, and a polarity may 

 result; but that polarity is not the polarity of the embryo. Oocyte polarity 

 may determine polarity of the blastocyst, that is, position of the embry- 

 onic area, which represents essentially a blastoderm; but whether it plays 

 any further role in determining developmental pattern is not certain. 

 Since embryonic dorsiventrality develops in a definite relation to the sur- 

 face-interior pattern of the embryonic area, the dorsal side being toward 

 the outer surface, it may represent either persistence of oocyte polarity 

 in the embryonic area or a direct reaction to surface-interior differences 

 or both. Since the apparently radially symmetrical blastocyst gives no 

 evidence of factors determining direction of the longitudinal embryonic 

 axis, the possibility suggests itself that this axis may be determined by 

 factors in the uterine environment, or, more specifically, in the region of 

 implantation. If a physiological differential is present in the uterine wall, 

 subjection of the embryonic area to it on contact with, or implantation 

 in, the uterine mucosa may perhaps determine that axis. Moreover, if 



