WOLFFIAN DUCT AND BODY IN THE CHICK. 439 
quently formed by the development of a lumen extending from 
the body cavity into the intermediate cell mass. 
The features of the stage of development now reached are well 
known ; it is that of the S-shaped cords of cells which have been 
so often described. In the adjoining woodcut is represented part 
of one of these S-shaped strings, showing clearly the above 
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Transverse Section through the Trunk of a Duck Embryo with about 
twenty-four Mesoblastic Somites. 
am. amnion ; so. somatopleure ; sp. splanchnopleure ; wd. Wolffian duct ; 
st. segmental tube; ca.v. cardinal vein; ms. muscle-plate; sp.g. spinal 
ganglion; sp.c. spinal cord; cA. notochord ; ao. aorta; Ay. hypoblast. 
features of a tubule, &c., viz.—(1) the Wolffian duct, in which a 
lumen has appeared ; (2) the outgrowth from it to the inter- 
mediate cell mass forming the upper limb of the S; (3) the 
