BIGELOW: EARLY DEVELOPMENT OF LEPAS. 129 
closing the blastopore. Ectoblastic cells around the blastopore give rise 
to mesenchyme (‘secondary mesoblast”), which forms most of the 
mesoblastic structures of the Nauplius. The mesoblast proper probably 
originates from the entoblast, as does the primary mesoblast of Lepas. 
It is evident that there is in Cyclops, according to Urbanowicz, a condi- 
tion closely resembling that of Lepas. 
In close agreement with Urbanowicz’s account of Cyclops and my 
own of Lepas, is Pedaschenko’s (’93) description of the formation of 
the germ-layers of the parisitic copepod Lernza. In this genus the 
mesoblast and ectoblast are separated from the yolk-entoblast in the 
first four divisions, as in Lepas. The four micromeres thus produced 
subdivide and form the blastoderm, which grows over the entoblast. 
At the margin of the growing blastoderm (blastopore) some cells (ap- 
parently ectoblastic) divide parallel to.the surface and form migrating 
mesenchyme cells. These apparently correspond to the “secondary 
mesoblast ’’ of Lepas. On the ventral side four of the cells sink beneath 
the ectoblast and constitute the primitive mesoblast cells. The lineage 
of these cells has not been definitely traced, but from their position I 
infer that they are probably the direct descendants of the fourth micro- 
mere, in which case the primary mesoblast originates directly from the 
entoblast, as in Lepas. 
Hiicker’s (’92, 97) studies of Cyclops led to results widely different 
from those of Urbanowicz. According to Hicker, a cell lying in the 
blastopore divides into a genital cell and a primitive mesoderm cell. 
The cells surrounding the blastopore divide, giving rise to the primitive 
endoderm cells; this is in line with Grobben’s account of Cetochilus, 
to which reference will be made later, and opposed to Urbanowicz, who 
found mesenchyme cells originating from cells bounding the blastopore. 
Grobben’s (’81) views of the formation of the germ-layers in the 
copepod Cetochilus do not agree with the account of Cyclops given by 
Urbanowicz, and only in part is there agreement with Hiicker’s account 
of Cyclops. His description of the thirty-two-cell stage of Cetochilus 
forms the best starting-point for purposes of comparison. In this stage, 
viewed from the vegetative pole, there is noticed a distinct bilateral 
symmetry in arrangement of the cells. A ‘central entoderm”’ cell and 
one small “ anterior entoderm” cell lie in the median plane. Four 
cells placed symmetrically on either side of the “central entoderm ” 
cell will by the next division form ‘entoderm” and ectoderm. The 
cell in the median line and posterior to the “central entoderm ” cell 
forms in later division four cells, of which the two nearer the “ central 
