676 PATTERNS AND PROBLEMS OF DEVELOPMENT 



mined, there must be factors in ovarian environment that can determine 

 a second physiological axis at right angles to the polar axis. Is there evi- 

 dence or probability of such factors? 



Oocytes of many animals are primarily cells of an epithelium. If this 

 epithelium is a part of the polar body gradient, as it is in various coelen- 

 terates, or if it develops progressively, as do the lobules of a branching 

 ovary or the ovarian rhachis and its epithelium in Ascaris, there is in it a 

 graded dififerential, at least during its development. This is parallel to 

 its surface, and the small fraction of this gradient in the oocyte is vertical 

 to a polar gradient between the free and the attached pole. Such a gra- 

 dient derived from the germinal epithelium may perhaps constitute the 

 basis of ventrodorsal pattern in some forms. Since the differential in the 

 single oocyte is very slight, the secondary axiate pattern may not be evi- 

 dent at the beginning of development and may appear gradually, as it 

 does in many animals. Even the tridimensional pattern of the Arenicola 

 egg (p. 658) may conceivably be determined in this way. The ovaries de- 

 velop in lateral regions of the body, and their cells share in anteroposterior 

 and ventrodorsal gradients of the body wall. If slight differentials in the 

 directions of these gradients are established in the oocyte at different 

 stages and a third arises between free and attached poles before the young 

 oocyte is isolated in the body cavity, a basis for the triaxiate pattern is 

 present. 



The ovarian tubules of insects and some other arthropods presumably 

 share, during their development, in the ventrodorsal gradient of the body. 

 If this is the case, the ventrodorsahty of the egg may perhaps represent 

 the persistence and further development of the fraction of this gradient 

 in the cell. Lateral asymmetries of some forms may originate in similar 

 manner from a differential in the parent body, involving the oocyte. 

 Certain dorsiventraUties and asymmetries of later development seem to 

 originate in this way. The temporary dorsiventrality of the Pdmatohydra 

 bud appears to be an expression of the longitudinal gradient of the parent 

 body (p. 634). The anteroposterior and dorsiventral patterns of the am- 

 phibian limb are apparently expressions in it of anteroposterior and dorsi- 

 ventral body gradients, and up to a certain stage its dorsiventrality can 

 be altered by altering orientation of the hmb bud to the body gradients. 

 Probably patterns of various other appendages of other animals are simi- 

 larly related to general body gradients. Dorsiventrality in embryos of 

 fishes, reptiles, and birds coincides in direction with the primary polarity 

 of the egg but may perhaps originate in the differential between surface 



