EVIDENCE PROVING THE STATEMENT OF THE CASE 93 



bodies again divide, giving four objects, and these four 

 again into eight objects, and so on from eight to 

 sixteen, to thirfcy-two, to sixty-four, and thus they in- 

 crease in geometrical progression. As they increase in 

 number they become smaller ; as they increase in num- 

 ber the whole mass of the egg increases in volume. 

 And thus pressing side by side, the spheroidal cells, 

 pressed together, become angular, attach themselves 

 to each other, and in this way is formed a cellular skin 

 or membrane. Membrane then overlaps membrane 

 until the fundamental compound membrane is formed. 

 It is a concentric formation like the layers in an 

 onion. 1 And from these concentric membranes, by 



1 ' The first step in the development of the mammalian ovum is 

 cell-multiplication; the first step in organisation is the regular 

 arrangement of its now numerous cells as a vesicle, the blastodermic 

 vesicle, with an accumulation of small granular cells at one pole 

 the future blastoderm. The first step in differentiation is the dis- 

 tinction visible between upper or outer and lower or inner ranks 

 of cells at this spot, soon followed by an obvious triple division 

 into upper or epiblastic lower or Jiypoblastic : and middle or meso- 

 blastic layers. This tripartite distinction is fundamental ; the 

 different tissues and organs in adult life are traceable back to 

 these three stocks respectively, and already in the three ancestral 

 membranes we may recognise in the germ a differentiation of 

 function that is preserved throughout life. The upper or external 

 layer is neural ; it will form epithelium the general epithelium of 

 the skin, the special epithelium of sense- organs, and, by involution, 

 the central axis of the cerebrum and spinal cord. The lower or 

 internal layer, lying on the yolk, is nutritive ; it will form the 

 digestive and absorbent epithelium of the intestine and intestinal 

 glands. The middle layer is the massive working layer, the future 

 bone and muscle of the body, and at a very early stage shows tokens 

 of the depuratory and excretory functions which are among its 

 functions in after-life ; it is the nidus of origin of the entire vascular 

 system and of the blood, and one of the earliest organs to appear in 



