EMBRYOLOGY OF THE KING-CRAB. 



321 



the serous membrane (Fig. 271, am), which is like that of 

 insects. 



Then the limbs bud out ; the six pairs of cephalic limbs 

 appear at once as in Fig. 271. Soon after the two basal 

 pairs of abdominal leaf-like feet arise, the abdomen be- 

 comes separated from the front region of the body, and 

 the segments are indicated as in Fig. 272. A later stage 

 (Fig. 273) is signalized by the more highly developed dorsal 

 portion of the embryo, an increase in size of the abdomen, 

 and the appearance of nine distinct abdominal segments. The 

 segments of the cephalo-thorax are now very clearly defined, 

 as also the division between the cephalo-thorax and abdomen, 

 the latter being now nearly as broad as the cephalo-thorax, 

 the sides of which are not spread out as in a later stage. 



Fig. 273. King-crab shortly before hatching ; trilobitic stage, enlarged ; side and 

 dorsal view. 



At this stage the egg-shell has split asunder and dropped 

 off, while the serous membrane, acting as a vicarious egg- 

 shell, has increased in size to an unusual extent, several 

 times exceeding its original dimensions and filled with sea- 

 Avater, in which the embryo revolves. 



At a little later period the embryo throws off an embry- 

 onal skin (amnion), the thin pellicle floating about in the 

 egg. Still later in the life of the embryo the claws are de- 

 veloped, an additional rudimentary gill appears, and the 

 abdomen grows broader and larger, with the segments more 



