46 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



separate. The part of the egg case attached to the membrane was often near to 

 the stalk of the egg case, which led one to infer that the attachment of egg case 

 and membrane and the making of the egg stalk might have some common 

 cause very early in the life of the egg, but no observations were made to de- 

 cide this. Left to itself a few minutes, such an embryo removed from the shell 

 burst the membrane without the aid of the egg case. Its telson was fastened 

 by its special glandular spines to the inside of the membrane, and soon it 

 reached about with its claws and seized hold of the membrane. This bag-like 

 membrane was thus the same thing as the telson thread of normal hatching. 

 When dissected from an embryo this membrane was seen to be a bag fast to 

 the telson spines, but with no observed special envelopes for the limbs as would 

 be the case were it a cast skin. Before hatching, the membrane was stretched 

 tightly all over the abdomen except where the two special groups of glandular 

 telson spines were, and there the membrane was raised up as two swellings, 

 one right and one left, much higher than the spines themselves. The space be- 

 tween the membrane and the spines was occupied by a mass of seemingly 

 liquid lumps which in some cases with 4 D showed a finely fibrous material 

 amongst the clear spines which, in places, extended out to the membrane like 

 a fibrous coagulum binding the spines to the membrane. On the median line 

 the membrane was close down against the telson so that the secreted masses 

 right and left seemed to have locally pushed the membrane away from its 

 original connection with the telson. 



This membrane was well developed in embryos of stages J to K of Reich- 

 enbach but upon dissecting, these embryos dropped out of the membrane, as 

 the telson was then not yet fast to the membrane, though the membrane was 

 firmly fastened to the egg case near the stalk. In the early stage F there was 

 also a membrane over the body and this was loose over the slightly projecting, 

 small abdomen. 



It seems probable that the telson thread of Astacus, which is a cuticle 

 formed over the embryo when its limbs are well advanced and thus has tubular 

 outgrowths to cover the limbs, is represented in Cambarus by a telson thread 

 having the form of a sac-like membrane formed so early that the small limbs 

 receive no special envelopes. In both cases there is a special outgrowth to cover 

 the abdomen, but while in Astacus this is a long bag, in Cambarus it is scarcely 

 recognizable as a separate region. 



In both, by some unknown process that is imagined to be associated with 

 fertilization phenomena, the membrane is made fast to the outer egg case, and in 

 both the membrane becomes fastened to the embryo by the activity of certain 

 telson glands. A diagram of Astacus would represent it as escaping from a 

 cuticle when hatching, a cuticle fastened to the egg case at one point, and near 

 that point fastened, inside, to the telson of the larva. A diagram of Cambarus 



