70 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The blastopore immediately closes up except in front, where it 

 acts as a mouth, and at this stage the swallowing of albumen 

 begins, and the embryo becomes distended and thin-walled as a 

 result (L, 0). 



3. Fate of the Germinal Layers. The ectoderm produces the 

 epidermis with its setigerous sacs, the nervous system, the neph- 

 ridia (except their funnels), and the epithelium lining fore- and 

 hind-guts. The mesoderm gives rise to nearly everything between 

 the epidermis and the epithelium of the gut, including circulatory 

 and reproductive organs, the funnels of the nephridia, and all the 

 muscular and connective tissue. The endodcrm originates the 

 epithelium of the mid-gut. 



Ectoderm. The fore-gut is formed at a comparatively early 

 stage as an ingrowth of ectoderm (st) at the unclosed mouth-end 

 of the blastopore (L, M). Soon after gastrulation cilia are devel- 

 oped along a median ventral strip of the ectoderm, and by their 

 action cause the embryo to rotate in the surrounding albumen. 

 The fore-gut is also ciliated. Not long before hatching the hind- 

 gut is formed as a small ectodermic involution which soon meets 

 and fuses with the mid-gut. 



At the time when the fore-gut is being formed a thickened 

 band can be made out running along each side of the body, and 

 known as a germinal "band. It is partly constituted by three 

 longitudinal rows of ectodermal cells, each of which is derived by 

 constant subdivision from a cell (teloblast) at its posterior end. 

 These rows and their teloblasts are at first superficial, but later 

 on sink in and are covered by other ectodermal cells. The middle 

 region of a fully developed germ-band (L, M, N) consists of three 

 layers 



(a) Outer layer, of ordinary ectodermal cells. 



(b) Middle layers, of cells derived from ectodermal teloblasts. 



(c) Inner layer of mesoderm. 



The internal teloblasts are known as neuroblasts (Nb), and the 

 neural cords (n.c) which they produce thicken and become the 

 right and left halves of the central nervous system, shifting down 

 and uniting ventrally to form the cord and uniting above the 

 mouth to form the nerve-ring (see P). The two external telo- 

 blasts on each side are termed nephroblasts (N), because the 

 nephridia (except funnels) grow out from the nephric cord (np.c) 

 which they originate. The two nephric cords on each side soon 



