428 PRINCIPLES OF EMBRYOLOGY 



through two phases in the prepupal and pupal stages, while L2 and the 

 posterior cross-vein belong to a different system, since they are not 

 represented in the prepupal wing; and for the posterior cross-vein in 

 particular we have clear evidence that it is determined at a later stage than 

 the longitudinal veins. Each system within the pattern must have a com- 

 plex epigenetic basis, since genes exist which can aker it in a number of 

 different ways. Sometimes the system reacts as a whole, as in dachs (where 

 the shape of the wing is important) or in shifted (where it is not) ; but 

 different parts of the system may have their own characteristic properties, 

 exhibited for instance in the localised effects of cubitus intermptus or tilt. 

 If a part of the pattern is removed at an early stage in the developmental 

 processes, compensatory phenomena may occur later. Finally we may 

 note that when the initial mass of material is much larger than normal, as 

 in blot, there is a tendency for the whole pattern to be dupHcated, the 

 duphcate being a mirror image of the normal. 



All these facts force one to conclude that a pattern represents a gradually 

 developing equiHbrium between a number of forces. The system of 

 forces may be very complex, and include a large number of different 

 items. This might suggest that it is almost hopeless to try to obtain any 

 fuller understanding of the genesis of a pattern. However, although the 

 total number of forces involved may be unmanageably large, it is quite 

 probable that only a few of them play major roles. For mstance, the wing 

 veins represent cavities wliich persist when the originally hollow sack of 

 the wing contracts and presses out the body fluid with which it was 

 originally fdled. It is clear that the tension in the wing epitheha, and the 

 hydrostatic pressure of the body fluid, must be among the important 

 factors with general effects on the process. T e other main element in the 

 situation is the set of factors which determine that there shall be only four 

 (and not perhaps five) longitudinal veins between the anterior and poster- 

 ior margins of the wing. It might not seem too optimistic to hope that 

 we could discover the nature of the processes which determine this major 

 feature of the pattern. It must be admitted, however, that as yet we have 

 scarcely any clue even as to the general type of phenomena which comes 

 into question. Are we dealing with diffusion, with the elastic and viscous 

 properties of the membranes, or with — ^what? 



(b) The pentadactyl limb 



As another example of an embryonic pattern we may briefly consider 

 the appearance of the bony structures in the vertebrate limb, hi the normal 

 hindlimb there is a single femur attached to the tibia and fibula, at the end 

 of which are the five digits. This fundamental pattern first appears as 



