BIGELOW: EARLY DEVELOPMENT OF LEPAS. 133 
lack of a definiteness of statement sufficient to afford basis for com- 
parisons of any value. 
Comparing the development of Astacus with that of Lepas, the ecto- 
blastic mesoblast at the anterior edge of the blastopore appears to be 
equivalent to the “secondary mesoblast” of Lepas. If the suggestion, 
that the invagination is mes-entoblastic, proves true, it may be possible 
to regard the mes-entoblastic cell d*:' of Lepas as representing the invagi- 
nated cells of the higher Crustacea ; the primary mesoblast and ento- 
blast of Lepas would then be comparable with the germ-layers derived 
from the invagination in the higher forms. In such a case there would 
be further agreement with Lepas in that the mesoblast originates from 
both ectoblast and entoblast. 
Summary. 
1. Lepas resembles most other Crustacea (a) in respect to position of 
the blastopore, which is ventral and posterior, () in extension of the 
entoblast and mesoblast from the blastopore as a starting-point, (c) in 
the mode of formation of the organs of the larva. 
2. In Lepas, as in most other Crustacea, the mesoblast and entoblast 
originate in the region of the blastopore from cells which, speaking in 
general terms, at first lie in the blastoderm and later migrate into the 
cleavage cavity. 
3. Among the migrating mes-entoblastic cells one can distinguish in 
Lepas the individual cells of entoblast and of two varieties of meso- 
blast. Representatives, if not precise homologues, of these kinds of 
cells are probably present both in other Entomostraca and in the higher 
Crustacea. 
XII. General Summary with Table of Cell-Lineage of Lepas. 
The results which are of special interest in relation to the develop- 
ment of Cirripedia have already been summarized in connection with 
the accounts of the several stages of development. Only results of 
more general interest are again summarized here. 
The cleavage of Lepas is throughout total and unequal. 
Stages with 2, 4, 8, 16, 32, and 62 “resting” cells are regularly 
formed. 
In the eight-cell stage and thereafter there is a well-marked bilateral 
arrangement of the cells. 
In the first three cleavages three “ protoplasmic”? micromeres are 
