628 Proceedings of Royal Society of Edinburgh. [sess. 
first laying down, at least in cases sueh as the present, where the 
germinal disc is regular and single. The series of sections shows 
clearly how the whole constitution depends on the relation of the 
axes to one another, and the form and disposition of the future 
organs can be definitely forecast. 
I do not purpose to enter in detail on the question of the ulti- 
mate origin of the duplicity. Professor Cleland (xvi.), in his 
notable contributions to the subject, regarded (1889) the cause as 
lying in a fission of the germinal mass occurring as the result of 
the application of some excitant, probably from without, the ulti- 
mate result depending on the date at which the fission takes place. 
It seems to he now practically universally admitted that a fission 
in some sense of the formative material does take place; but recent 
research all tends to prove that by the time the germinal layers 
are laid down, the cells have acquired their specific characters, 
and are incapable of dividing to form two individuals of the same 
potentiality.* This is supported by the striking fact that in birds 
duplicity has not yet been produced artificially. Gerlach (ii.) 
sought by varnishing eggs, except over a Y-shaped area above the 
blastoderm, to produce a fission of the germinal mass, but his 
results are universally regarded as proving nothing. Moreover, 
considerable light has, within recent years, been thrown on the 
possible nature of this fission by the experiments of the Entwicke- 
lungs-mechanik school, associated with the names of Roux, 
Driesch, Morgan, Wilson, Loeb, Herlitzka, 0. Schultze, and 
others mentioned in the literature list appended. By various 
methods double and monstrous forms have been artificially pro- 
duced from a single egg. 
All these experiments show that while in normal circumstances 
each blastomere has its fixed share in the developmental process, 
under conditions of separation, of altered mutual relationship, 
or of disturbed equilibrium, they may revert to the condition of 
the entire ovum, segment as it does, and produce double, or if 
separation is incomplete, monstrous, forms. 
* Assheton (xxix. ) thinks it conceivable that irregular splitting may some- 
times arise in the morula stage, and during the formation of the blastodermic 
vesicle, in such a way that the embryonic mass becomes divided into two 
portions. 
