03 THE FACTORS OF ORGANIC EVOLUTION. 



Here, instead of dwelling on these details, it will bo 

 better to draw attention to the most general aspect of the 

 facts. Whatever may be the course of subsequent changes, 

 the first change is the formation of a superficial layer or 

 blastoderm ; and by whatever series of transformations 

 the adult structure is reached, it is from the blastoderm 

 that all the organs forming the adult originate. Why this 

 marvellous fact ? 



Meaning is given to it if we go back to the first stage in 

 which Protozoa, having by repeated fissions formed a clus 

 ter, then arranged themselves into a hollow sphere, as do 

 the protophytes forming a Volvox. Originally alike all over 

 its surface, the hollow sphere of ciliated units thus formed, 

 would, if not quite spherical, assume a constant attitude 

 when moving through the water ; and hence one part of 

 the spheroid would more frequently than the rest come in 

 contact with nutritive matters to be taken in. A division 

 of labour resulting from such a variation being advanta 

 geous, and tending therefore to increase in descendants; 

 would end in a differentiation like that shown in the gem- 

 mules of various low types of Metazoa, which, ovate in shape, 

 are ciliated over one part of the surface only. There would 

 arise a form in which the cilium-bearing units effected loco 

 motion and aeration; while on the others, assuming an 

 amoeba-like character, devolved the function of absorbing 

 food : a primordial specialization variously indicated by 

 evidence.* Just noting that an ancestral origin of this 

 kind is implied by the fact that in low types of Metazoa 

 a hollow sphere of cells is the form first assumed by the 

 unfolding embryo, I draw attention to the point here of chief 

 interest; namely that the primary differentiation of this 

 hollow sphere is in such case determined by a difference 

 in the converse of its parts with the medium and its 

 contents ; and that the subsequent invagination arises by a 

 continuance of this differential converse. 



* See Balfour, Vol. i, 149 and Vol. ii, 313-4. 



