64 THE FACTORS OP ORGANIC EVOLUTION. 



forms a membrane extending over the whole surface the 

 blastoderm. The next stage of development, which ends 

 in this covering layer becoming double, is reached in two 

 ways by invagination and by delamination; but which is the 

 original way and which the abridged way, is not quite cer 

 tain. Of invagination, multitudinously exemplified in the 

 lowest types, Mr. Balfour says : &quot; On purely a priori grounds 

 there is in my opinion more to be said for invagination 

 than for any other view&quot;;* and, for present purposes, it 

 will suffice if we limit ourselves to this : making its nature 

 clear to the general reader by a simple illustration. 



Take a small india-rubber ball not of the inflated kind, 

 nor of the solid kind, but of the kind about an inch or so 

 in diameter with a small hole through which, under pressure, 

 the air escapes. Suppose that instead of consisting of india- 

 rubber its wall consists of small cells made polyhedral in 

 form by mutual pressure, and united together. This will 

 represent the blastoderm. Now with the finger, thrust in 

 one side of the ball until it touches the other : so making a 

 cup. This action will stand for the process of invagination. 

 Imagine that by continuance of it, the hemispherical cup 

 becomes very much deepened and the opening narrowed, 

 until the cup becomes a sac, of which the introverted wall 

 is everywhere in contact with the outer wall. This will 

 represent the two -layered &quot;gastrula&quot; the simplest 

 ancestral form of the Metazoa: a form which is permanently 

 represented in some of the lowest types ; for it needs but 

 tentacles round the mouth of the sac, to produce a common 

 hydra. Here the fact which it chiefly concerns us to 

 remark, is that of these two layers the outer, called in 

 embryological language the epiblast, continues to carry on 

 direct converse with the forces and matters in the environ 

 ment ; while the inner, called the hypoblast, comes in contact 



A Treatise on Comparative Embryology. By Fiaucis M. Balfour, LL.D, 

 r.ir.s. Vol. ii, p. 313 (second edition). 



