126 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
merely suggestions which have grown out of comparison of the order of 
the cleavages involved in segregating the germ-layers. 
The segregation of the ectoblast as three micromeres is apparently 
not peculiar to Lepas among Entomostraca. The cleavage of certain 
parasitic Copepoda has close resemblances to that of Lepas as regards 
number of cleavages involved in the segregation of the germ-layers. 
In Lernzea, according to Pedaschenko (93), the ectoblast and mesoblast 
are separated from the yolk-macromere (entoblast) by means of four 
cleavages. It will appear in the discussion of the germ-layers in the 
following section of this paper, that in the instance just cited the first 
three micromeres probably contain all the ectoblast with the ‘‘second- 
ary mesoblast,” while the fourth is the primary mesoblast ; in this case, 
then, the number and order of cleavages involved in germ-layer segrega- 
tion would agree with my observations on Lepas. 
In the figures and accounts of the cleavages of various phyllopods and 
copepods, in which the germ-layers appear to be established as early as 
the thirty-two-cell stage, there are found many suggestions that further 
investigations may show a close resemblance to the cell-lineage of Lepas. 
Some examples of such suggestive papers are those of Grobben (79, ’81) 
on Moina and Cetochilus, Urbanowicz (’86) and Hacker (92, 97) on 
Cyclops, and Pedaschenko (’93) on Lernza ; but in none of these genera 
are the facts as yet sufficiently well known to warrant close comparison 
with Lepas, especially since there is much disagreement between the 
observations of these investigators. At present this mention of a possi- 
ble resemblance to the cleavage of Lepas can have only the value of a 
suggestion, which may possibly stimulate comparative study of the 
cleavage of those Entomostraca in which the early segregation of the 
germ-layers makes it possible to trace the lineage of the cells to the com- 
plete separation of the germ-layers. 
The cleavage of Lepas has some general resemblances to that of the 
nematodes. Particularly is there resemblance in the early segregation 
of the germ-layers; but, as to the order of cleavage involved in this 
process, there are great and at present irreconcilable differences. The 
first cleavage in Nematoda begins the separation of the germ-layers. 
Thus the cell ad? contains ectoblast in the nematodes as in the cirri- 
pede, and ed? contains ectoblast and mes-entoblast. The second cleay- 
age in the nematodes completes the segregation of the mes-entoblast 
from ectoblast, whereas this is accomplished by the third cleavage in 
Lepas. It is obviously impossible to make any comparison of the 
details of the early development. 
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