ON THE ORIGIN OF VERTEBRATES PROM ARACHNIDS. 375 



germ layers^ and it certainly is not a gastrula, although in 

 surface views it is suggestive of such an organ. ^ 



In Limulus there is a remarkable ring of mesoderm cells 

 extending completely around the embryo. It may be seen 

 in surface views as two dark bauds diverging from the region 

 behind the primitive streak, and uniting with each other 

 in the neighbourhood of the oesophagus (Fig. 18, a). In 

 sections the bands appear as two clusters of oval cells, 

 formed by an inner proliferation of the ectoderm (Fig. 18, 

 c and D, m.b.). They are soon freed from the overlying 

 ectoderm, and finally, after increasing enormously in size, 

 meet each other along the median dorsal surface. By the 

 time the bands are well formed the cells are oval, and contain 

 an enormously long, brilliantly refractive filament, which is 

 either wound or bent back and forth a great many times. The 

 small remaining space in the cell is filled with a watery fluid, 

 and a small, laterally placed nucleus. Some of the cells 

 become elongated, and the coiled fibres give rise to the strise 

 of the longitudinal dorsal muscles. Some of the cells appear 

 to degenerate and disappear. The resemblance of this ring, 

 at an early stage, to the '' Keimwall " of Vertebrate embryos 

 need not be enlarged upon here. I have not with certainty 

 found anything similar in Scorpions. 



The following expresses what I conceive to be the most 

 natural arrangement of the segmented animals : — 



^ This condition is probably produced by some peculiarity in the division 

 of the cells of the posterior end of the body. All foldings of cellular mem- 

 brane are due, I believe, to local variations in cell division. Wherever a cell 

 layer tends to increase in thickness by tangential division a simple thickening 

 will be produced. But if the cells thus produced at the same time multiply 

 by radial division, the inner surface of the membrane will be larger than the 

 outer, and the whole membrane curved or warped. The direction of 

 curvature will depend entirely on the relative rapidity of division at certain 

 points. All foldings may then be regarded as the expression of certain 

 methods of cell-growth, and may not have themselves any morphological 

 significance. 



