Tissues in the Crustacean Limb. alg bl 
progress of the present work. I am also indebted to Professor 
A. D. Mead, of Brown University, for generously permitting me 
the use of materials and apparatus at the Experiment Station of 
the Rhode Island Commission of Inland Fisheries; and to Professor 
R. J. Terry, of Washington University, for valuable suggestions 
during the completion of the present paper. 
Il. Materirat anp Mrtuops. 
A serious obstacle encountered in determining the origin and dif- 
ferentiation of a regenerating tissue is the difficulty of securing a 
complete series of successive developmental stages. If a number 
of animals are operated upon at the same time and material fixed 
at successive intervals, it is usually found necessary in subsequent 
study to make an extensive rearrangement of the material on account 
of variations in size and rate of growth. These technical difficulties 
were considerably lessened in the present work because of the excel- 
lent material obtained at the lobster hatchery of the State Experiment 
Station at Wickford, Rhode Island. From a hatchery pool contain- 
ing several thousand lobster fry, it was possible to select a large 
number of animals which were practically equal in age and size, 
and which had all moulted to the same stage within less than twenty- 
four hours. This last point is of considerable importance, since 
it had been ascertained in previous experiments (’06) that the age 
of the animal and the time, in relation to moulting, at which the 
operation is made are important factors influencing the rate of regen- 
eration. These young lobsters were also further favorable for study 
because the exoskeleton had not as yet attained such an amount of 
chitin and lime salts as to necessitate decalcification before sectioning. 
The operation consisted in the autotomous removal of both chelze. 
The lobsters, about 300 in number and all practically equal in age 
and size, were placed in floating cars and kept in as nearly normal 
condition as possible. Specimens were then preserved at two, four, 
and eight hour intervals throughout a period of fourteen days, in 
which time the chele had undergone complete regeneration. From 
this succession of stages about 54 were selected, sections of the chele 
were cut 5 micra in thickness and mounted in series. In order to 
