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PRINCIPLES OF EMBRYOLOGY 



Operations one allows the limb to develop and examines whether a disc 

 whose antero-posterior axis was reversed actually develops back to front, 

 in which case the axis may be said to have been already determined, or 

 whether influences from the host's body succeed in causing a reversal of 

 polarity within the limb-disc (Fig. 12.10). 



It was soon discovered that the various axes are determined at different 

 times. The antero-posterior one always becomes fixed first. According to 

 Detwiler it is already determined by the middle gastrula stage, but 

 Takaya gives reasons for doubting this, and suggests that the determina- 

 tion does not actually occur until the neural plate stage. Even so, this is 

 much earlier than the determination of the dorso-ventral axis, which does 



Figure 12.10 



On the left, an axolotl embryo into which a hmb-bud has been grafted 

 with its dorso-ventral and anterior-posterior axes reversed. The transplant 

 [Tr) has developed into a limb with reversed anterior-posterior axis but 



normal dorso-ventral axis. (After Harrison.) 



On the right, a newt embryo showing the direction of the anterior-posterior 



field in different regions of the flank. The circles mark the normal positions 



of the limb-buds. (From Takaya 1941.) 



not occur till the fairly late tail-bud stage, by which time the limb-buds 

 have already become slightly elevated from the side of the body. It is 

 shortly after this that the medio-lateral axis of the mesodermal com- 

 ponent of the limb-bud becomes also determined. 



The determination is certainly brought about by the tissues in the im- 

 mediate neighbourhood of the disc. This can be shown by rotating a 

 fairly large area in the limb region and then grafting into this region a limb- 

 disc, with one or more of its axes reversed. In such cases it is the orienta- 

 tion of the immediately surrounding area rather than that of the whole 

 host embryo which is decisive over the future development of the limb. 



The nature of the influence which fixes the polarity of the limb-disc is 

 still imperfectly understood. It clearly falls into the general category of 

 what have been referred to as 'field characters', but that terminology does 

 not tell us much about what it actually is. Harrison (1936) thought that 

 the polarity might depend on some fme-grain structure of the tissue of 



