i 9 8 THE POPULAR SCIENCE MONTHLY. 



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 certain. 

 Of invagination, multitudinously exemplified in the lowest types, Mr. 

 Balfour says : " On purely d priori grounds there is in my opinion 

 more to be said for invagination than for any other view " ; * 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 in- 

 stead 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 open- 

 ing narrowed, until the cup becomes a sac, of which the introverted 

 wall is everywhere in contact with the outer wall. This will repre- 

 sent the two-layered "gastrula" 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 environment ; while the 

 inner, called the hypoblast, comes in contact with such only of 

 these matters as are put into the food-cavity which it lines. We 

 have further to note that in the embryos of Metazoa at all ad- 

 vanced in organization, there arises between these two layers a third 

 the mescblast. The origin of this is seen in types where the devel- 

 opmental process is not obscured by the presence of a large food-yolk. 

 While the above-described introversion is taking place, and before the 

 inner surfaces of the resulting epiblast and hypoblast have come into 

 contact, cells, or amoeboid units equivalent to them, are budded off 

 from one or both of these inner surfaces, or some part of one or other ; 

 and these form a layer which eventually lies between the other two 

 a layer which, as this mode of formation implies, never has any con- 

 verse with the surrounding medium and its contents, or with the nu- 

 tritive bodies taken in from it. The striking facts to which this de- 

 scription is a necessary introduction, may now be stated. From the 

 outer layer, or epiblast, are developed the permanent skin and its out- 

 growths, the nervous system, and the organs of sense ; from the intro- 



* A Treatise on Comparative Embryology. By Francis M. Balfour, ll. d., f. r. s. Yol. 

 II, p, 343 (second edition). 



