112 Part III. — Tivcnty-second Annual Report 



The plumose hairs are not confined to the two edges of the exopo- 

 dite, but are also found on the outer surface, vid. fig. 42. They do not, 

 however, run round the stem in rows as do the hairs of the endopodite ; 

 they are simply scattered over the outer surface. 



The inner surface of the exopodite, fig. 51, is provided with scattered 

 short hairs, which are serrated. 



The Ripe Egg. 



The eggs of Cancer pagurus are ripe during October, November, 

 December, and January,* and spawning may take place in each of these 

 months. The eggs are extruded in a short space of time, probably within 

 a period of twenty-four hours. 



In my former paper, " Contributions to the Life-History of the Edible 

 Crab {Cancer pagitriis)," I described the ripe ovary as follows : — " The ripe 

 ovary is of a turkey-red colour. . . . All the eggs are not of one size. 

 The diameter of the yolk-mass may vary from "3 — '41 mm.; in some eggs 

 the yolk-sphere is as small as "24 mm. The diameter of the Zona 

 radiata varies greatly from the fact that the egg in the ovary has a large 

 perivitelline space. . . . The diameter of the capsule may vary from 

 •4 — '7 mm. ; the eggs attached to the swimmerets measure "45 and '5 

 mm. in diameter." I have, however, come to the conclusion that the 

 condition just described, where the ovarian egg shows a large peri- 

 vitelline space, is a pathological one. I have since then only found it in 

 crabs that died during the spawning season ; the dropsical condition of 

 the ovary having possibly been the cause. 



The ripe ovary, however, sometimes exhibits .a condition which 

 suggests the presence in it of eggs with large perivitelline space. In a 

 crab measuring 7^ inches across (17 November, 1903) the ovary was full 

 and of a crimson-red colour. When its outer surface was examined with 

 a lens, a clear area was seen surrounding the egg. This clear area is a sort 

 of fluid space in the follicle, and is not a perivitelline space ; it is 

 outside the egg. 



The ripe egg has tioo envelopes — the inner, the vitelline membrane 

 (v.m.), is clearly applied to the yolk-sphere ; the outer, the chorion (chr.), 

 is separated from the former by a very narrow space when the egg is 

 in ovario. Fig 5 shows a section of the ripe ovarian egg. It is contained 

 in the follicle (/.). The yolk-sphere is composed of large corpuscles. 

 Mayer t was of the opinion that fertilisation took place in the ovary 

 before the egg was invested with the chorion. 



Rathke J described, on the egg of Astacus, three egg-membranes, viz. 

 "die Dotterhaut" [the vitelline membrane], "die Lederhaut," and "die 

 aussere Eihaut" [the chorion]. In the egg, previous to the commence- 

 ment of the development of the embryo, there is a space between the 

 " Dotterhaut " and the " Lederhaut," which contains a transparent fluid ; 

 the quantity of this fluid diminishes as the development proceeds. In 

 this way the " Dotterhaut " and the '"Lederhaut" come to lie closely 

 together. The "aussere Eihaut" is that by which the egg is attached to 

 the swimmeret. This description does not apply to Cancer pagurus, 

 where there are only ttvo egg-membranes. 



A section of a dropsical ovarian egg is seen in fig. 34. These eggs 

 can be made out with the naked eye scattered over the surface of a lobule 

 of the ovary when few in number ; when the majority of the eggs are 



* Heath's observations lead to a similar spawning-period for Cancer iruigister on the 

 coast of California. American Naturalist, 1902. 

 f P. Mayer, Jena, Zeit. Naturivissen, 11 Bd., 1877. 

 t Rathke, 



