(26) 
B , From these two have arisen many cells, which are now spread 
as two membranes. C, The multiplication of cells has continued, 
and between the two membranes first formed a third membrane 
has arisen, and this one splitting into two, on each side, close to 
the middle line, four membranes result; by the thickening and 
folding of which in various ways, the subsequent embryo is formed. 
Fig. 12. Transverse section of yolk at stage more advanced 
than C, Fig. 11. The cellular membranes are represented by the 
curving lines, i. Spinal-cord tube, neatly closed. 2, Digestive 
canal. 3, Cavity soon to be closed in, as chest and abdomen. 
4, Yolk-cavity. 
Fig. 13. More advanced stage of Fig. 12. The numbers 
refer to same parts as in the preceding figure. 
