BIGELOW: EARLY DEVELOPMENT OF LEPAS. 123 
was evidently based upon Nassonow’s figures of Balanus ; but is shown 
to be erroneous by subsequent investigations. It is controverted in the 
case of Balanus, by the account of Groom, as well as by unpublished 
observations of my own; and in the case of Lepas it is clearly inappli- 
cable. In both these genera cleavage is total and unequal. 
Knipowitsch (’92) described the cleavage of the Ascothoracidan genus 
Laura as superficial from the very beginning of development. His figures 
do not warrant such a conclusion, for cell-boundaries appear to form after 
every nuclear division. The few figures of segmentating eggs in Knipo- 
witsch’s paper resemble the figures which other authors have drawn from 
the eggs of parasitic copepods ; for example, Pedaschenko’s (’93) figures 
of Lernza. The latter is evidently a case of total, but very unequal, 
cleavage, and the cleavage of Laura is apparently to be interpreted in 
the same way. 
Van Beneden’s (’70) figures illustrating his account of the develop- 
ment of Sacculina indicate to my mind that the cleavage of Rhizoce- 
phalan Cirripedia is also of the unequal total type. Even the fact that 
in late stages the four yolk-macromeres appear to fuse does not support 
the interpretation that the cleavage is in later stages superficial. In no 
stage of the development is there nuclear division which is not associated 
with total cell division, and we are led to the conclusion that the cleavage 
of Sacculina cannot be correctly characterized as superficial in any stage. 
Regarding the type of cleavage of cirripede ova, the conclusion is that, 
so far as present knowledge extends, the eggs undergo unequal total 
cleavage, and with respect to the cleavage processes there is no close 
resemblance to the superficial cleavage of the higher Crustacea ; rather 
is the resemblance to that of the yolk-laden eggs of gasteropods. 
In the order of the cleavages involved in the establishment of the 
germ-layers there are in Lepas some interesting resemblances to the 
annelids and mollusks. As is well known, studies of the cell-lineage of 
annelids, gasteropods, lamellibranchs, and chitons have shown that in 
all of these forms the ectoblast is separated from the mes-entoblast by 
three successive cleavages, while a fourth cleavage separates the primary 
mesoblast from the entoblast. Moreover, it has been shown in the cases 
of some gasteropods and lamellibranch mollusks, that the mesoblast is 
derived from both primary germ-layers; in addition to the primary 
mesoblast (entoblastie mesoblast) there are mesoblast cells which come 
from the ectoblast (ectoblastic mesoblast). This has been designated 
“‘secondary mesoblast”’ or “larval mesenchyme’ (Lillie, ’95, p. 24; 
Conklin, ’97, p. 150). 
VOL. XL.—2 _ 5 
