No. 2.] ANURIDA MARITIMA. 253 
holoblastic cleavage and eventually lose it. Brauer’s ('92) 
figures are given for Branchipus, in which the change takes 
place in the following way: holoblastic cleavage results in a 
distinct and regular cleavage cavity. This method continues 
until a layer of small, narrow, and very long columnar cells is 
formed, having their nuclei on the periphery and long yolk 
stems extending in to the centre. Eventually, this inner part 
forms a fused yolk mass with a single layer of blastoderm cells 
on the outside not separated from the yolk. Alpheus, Palae- 
monetes, and Hippa all have a similar cleavage according to 
Herrick. Ishikawa (’85) states that in a fresh-water form some 
cells remain behind; these are, he thinks, doubtful in signifi- 
cance. Among the pantopods there are two methods of cleav- 
age. Morgan ('91) describes the cleavage in Pallene as at first 
total; gradually the nuclei become more and more peripheral, 
and eventually the cleavage planes in the yolk mass are lost. 
Tanystilum and Phoxichilidium present another variation; holo- 
blastic cleavage is maintained up to the 16-celled stage with 
the formation of a central cavity (Figs. IX, X). This is 
followed by a delamination of the entoderm from the inner 
ends of the single layer of cells (Fig. X, A, #), filling the 
blastocoele with entoderm cells. In this way entoderm arises 
by what might be considered a process of multipolar migration. 
Among the myriapods certain phases are suggestive. Zo- 
graff (90) figures Geophilus as having purely centrolecithal 
cleavage of the nucleus, accompanied by a certain kind of yolk 
cleavage (Fig. XI, A, B). This gives the outward appearance 
of holoblastic cleavage that has been claimed as general 
throughout the group. The central nuclei now migrate out- 
wards along these yolk cleavage lines, and the blastoderm is 
formed by migration. Small masses of cells are found at the 
ends of each cleavage line that eventually spread out and form 
a complete blastoderm (Fig. XI, D). 
It is clear from these few cases that change in method of 
cleavage is a widespread fact among arthropods, but accom- 
plished in many different ways. Anurida differs from any of 
the others described. As in Branchipus no blastocoele is 
formed, the strictly one-layered condition is early lost, and a 
