EXPLANATION OF THE PLATES. xxill 



separation, has now "become very strong, covers the young 

 germ, thus acting as a means of defence from external injury. 

 Figs. 3, 4, 5, Are the same parts in progressive states of development. 

 Fig. 5 presents a bifurcated character ; probably from some 

 accidental cause it thus appears smaller than it is in the 

 normal state. 



Fig. 6 Eepresents the raw surface of the leg, already alluded to in 

 Carcinus mcenas, some time after separation. A nucleated 

 cell is seen in the centre. This drawing was made from a 

 very small specimen, and was only procured in the stage re- 

 presented after great difficulty. 



Fig. 7 Represents a longitudinal section of a very young germ, for 

 the purpose of showing its mode of development. The fibrous- 

 looking band which surrounds it externally, is a circular canal 

 which belongs to a system of vessels described in the text. 

 The four striated bodies which lie next to this canal are the 

 rudiments of the four joints of the future limb. The striated 

 appearance arises from the muscles already so far developed, 

 and the albuminous matter within, and which they enclose, 

 appears to be pabulum for their farther nourishment. The 

 more defined globules, which may be observed floating 

 amongst the albumen, are oil-globules. In the development 

 of this leg, it will be observed that the external segments, or 

 those which are analogous to the thigh and first tibial joints, 

 are largest and most fully formed, a fact we would be led 

 to expect, from the circumstance of their formative cells being 

 the first thrown off from the original parent nucleus, and 

 consequently the first that would take on a central or more 

 independent action. From a similar mode of development, 

 we see that the second tibial and tarsal joints are the smallest, 

 as they are the last formed of the centres. The last or distal 

 phalanx is the smallest of the internal segments ; those 

 nearest the circular vessel are the largest, as was to be ex- 

 pected from the centres which formed them, being the oldest 

 and the first formed from the earlier generations of cells ; and 

 those again within them are smaller, being formed from the 

 later generations thrown off by the original parent. 

 Fig. 8. Cells from the external series represented by c in Fig. 9. 

 Fig. 9. Transverse section of raw surface of proximal or attached ex- 

 tremity of the reproductive organ in leg of Cancer pagurus. 

 This is the surface and appearance which is seen immediately 

 upon the leg falling off ; if it is seen half-an-hour, or a little 

 more, after the separation, it is covered with a thickish film, 

 which shortly becomes a strong opaque cicatrix hiding every- 

 thing beneath it. The vessels seen in Fig. 15 are also 

 omitted, for the purpose of showing the structure of the re- 

 productive body more clearly. 



