NATURAL HISTORY OF AMERICAN LOBSTER. 3II 



Before the question of egg attachment in the crab can be settled we must have very 

 full and exact observations of the behavior of these animals during the period of egg 

 laying. Now in Callinectes the endopodites are packed full of " cement " or tegumental 

 glands; the exopodites contain fewer glands but an excess of cell disks or concretions 

 (see 149, p. 108). In fact, Braun called attention to the presence of glands in these 

 Brachyura over thirty years ago. 



If the secretion of the receptaculum seminis of the crabs is limited only to the uses 

 of the sperm, as seems probable, we are inclined for the present to accept the older 

 theory, namely, that eggs are glued to the hairs by a cement which is secreted by glands 

 which lie at their base. 



Why the eggs of the Callinectes are not stuck together or why neighboring hairs do 

 not more frequently adhere is not apparent, and can not be explained until we know 

 more about the physical properties of the glue itself. The hairs of Callinectes are 

 covered with a continuous sheet of glue, but are not often adherent. Possibly the eggs 

 stick to them before they have a chance to become entangled. Each egg is tethered by 

 a thin spun sheet of glue, which is continuous with a narrowband or sheet, in which the 

 entire hair is embraced up to the tip or very close to it. 



As was pointed out by the writer in 1892 and as had already been demonstrated by 

 Mayer in 1877, the crustacean egg does not possess a yolk-membrane. The ovarian 

 ovum and the mature egg when it issues from the ovary, in crustaceans as well as in 

 insects, is provided with a single membrane, the chorion, which is secreted by the "ovi- 

 sac" or egg follicle. The great mass of the egg is made up of inert yolk; the protoplasm, 

 which alone has formative power, is practically restricted to the center of the egg. When 

 in the course of segmentation or later the protoplasm has reached the surface, a delicate 

 membrane is secreted by the blastoderm. This often glues the egg fast to the chorion 

 and gives much trouble to the embryologist. No doubt it was this membrane which 

 gave rise to the mythical " Dotterhaut, " or vitelline membrane of Erdl, Rathke, and the 

 older school of embryologists. 



A single membrane only, the chorion, is apparent in the eggs of Callinectes, but since 

 the cord of attachment spreads out over its surface without any apparent break, the egg 

 is probably covered with a thin layer of cement which has the same index of refraction 

 as the chorion to which it is inseparably glued. 



Williamson endeavors to extend his ingenious theory of fixation by "spearing" and 

 the liberation of the cement from the egg itself to the lobster and other ]\Iacrura. Thus 

 he says that the secretion "is not a true cement" capable of forming an outer envelope, 

 but an albuminous substance, and that "the weight of the egg tends to stretch out the 

 ductile chorion into long thin stalks." It is quite certain that the egg of the lobster, as 

 in all the higher Crustacea, possesses a single membrane when it leaves the ovary, but 

 the egg attached to the body has acquired a second and distinct membrane which is 

 continuous with the stalk of attachment. The two are easily separable in picro-sulphuric 

 acid; the second or outer layer is the "cement membrane" (fig. 5, pi. XLiv). 



As we have already seen (p. 305) the eggs of the lobster are attached to the non- 

 plumose hairs of the swimmerets as well as to the abdomen and to each other. Here at 



