Polarity in Organic Elements. 369 



more or less distinctive character. We must conclude that in each case 

 some slight difference of composition in these units leading to some 

 slight difference in their mutual play of forces, produces a difference in 

 the form which the aggregate of them assumes." (Spencer.) 



" It is well to recollect that an organic particle ^i^ of an inch in 

 diameter in which our best microscopes may be incompetent to reveal 

 the slightest differentiation of parts may be made up of 1,000,000 par- 

 ticles 100 * 000 of an inch in diameter, while the molecules of matter are 

 probably much less than roo * 000 of an inch in diameter. Hence in such 

 a body there is ample scope for any amount of complexity of molecular 

 structure." (Huxley Invert Anat. 15. ) 



A large part of the movement of the early embryo takes the form of 

 invagination and evagination, an apparent pushing in and out of the 

 various membranes. The manner in which the entoderm takes the place 

 as the lining of the ectoderm is direct invagination, most clearly recog- 

 nized in the case of the Bell-gastrula, from which the human Hood- 

 gastrula is, however, developed, and in which the process is still really 

 invagination. If a short stick were to be pressed sidewise against the 

 side of a soap bubble till it was buried, the membrane of the bubble 

 coming up on the sides and ends and folding over till the folds touched 

 on top, the process would resemble the formation of the medullary tube, 

 the stick being supposed to represent the position of the tube when 

 formed. The sinkings and swellings of the several parts of the mem- 

 brane represent alternate imaginations and evaginations. So the for- 

 mation of many of the other parts is in imitation of the same process. 

 The optic vesicle evaginated from the brain bladder, the cavity of the 

 ovo ball and vitreous humor invaginated from the outside, the invagin- 

 ation of the ear sac, of the mouth opening, the nostrils and the anus, 

 the evagination of the tail, the limbs, etc. , are other examples. And 

 so are the sinking of the whole embryo down into the amnion sac, and 

 the sinking of the whole embryo back into the skin layers by which pro- 

 cess the yelk sac is pinched in two and the body cavity formed. This 

 pushing in and pushing out, however, are not the real processes', but 

 only the apparent results of other processes. As we saw in stud}'ing 

 the segmentation of the parent egg, the segmentation of one set of cells 

 at first goes on faster than the segmentation of the other. As each cell 

 duplicates itself it simply spreads and extends the membrane of which 

 it is a member. As the two membranes are attached to each other and 

 one growing faster than the other, the only possible result is that the 

 faster one will soon enclose the other, or invaginate it. But the process 

 of rapid growth alternates from one set of cells to another. The energy 

 of growth temporarity exhausting the capacit} 7 of one membrane for ex- 

 pansion, is transferred to another, while the first one pauses. It is like 



