122 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the longitudinal furrow laterally and ventrally is shown in Figure 94, 
which also shows the ectoblast and mesoblast composing the appendages. 
The deepening of the furrows progresses and the appendages are folded 
off commencing at their dorsal distal ends until finally their attachment 
is to the ventral side of the embryo, as determined by the position of the 
mouth and labrum (Figs. 124, 126). It will be seen that my account 
confirms Groom in that the mesoblast band and the furrows are dorsal, 
and that the appendages are folded off from dorsal to ventral, the free 
ends of the appendages remaining directed dorsally until about the time 
of hatching. Investigators before Groom gave good de&criptions and 
figures of the formation of appendages, but considered that the meso- 
blastic band and the furrows were ventral instead of dorsal. 
Many of my preparations and unpublished figures of later stages con- 
firm Groom’s account regarding the formation of the stomodzeum and 
proctodeum, and the development of the mesenteron from the yolk- 
entoblast cells. 
It is to be noted that many of Groom’s minor observations on later 
stages were confirmatory of earlier writers, whose work he has reviewed, 
and it has, therefore, for my purposes been sufficient to refer directly to 
Groom’s paper. For the details of late development of organs of the 
Nauplius, reference must be made to Groom and earlier workers, for this 
paper is concerned, primarily, with cleavage and germ-layer formation. 
The fate of the germ-layers, which were identified in the sixty-two-cell 
stage, may be summarized as follows : — The ectoblast forms the outer 
covering of the body and appendages, the stomodeeum, proctodeeum, and 
the nervous system. The yolk-entoblast forms the mesenteron. The 
mesoblast forms the muscles and connective tissues of the appendages, 
and of the body of the Nauplius. 
So far it has not been possible to distinguish between the fate of the 
primary and secondary mesoblasts. It can only be stated that at least 
a part of the muscular and mesenchymatous tissues of the Nauplius come 
from the ecto-mesoblast (‘secondary mesoblast”). In other genera of 
Cirripedia an attempt is now being made at tracing the two kinds of 
mesoblast farther than has been possible in Lepas. 
X. General Considerations on Cleavage and Cell-Lineage. 
Korschelt und Heider (’90-91) have classed the cleavage of the cirri- 
pede ovum with their type II of crustacean cleavage — a type beginning 
with total cleavage, but soon changing to superficial. This classification 
