128 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
sal surface, the conditions in Lepas would be closely imitated. The 
mesoblast bands would in such a case come to lie more and more dor- 
sally, in proportion as the loss of yolk allowed the embryonic disk to 
cover the whole surface. In Lepas these bands in their position near the 
median-dorsal line, where the distal ends of the appendages later appear, 
may be considered as representing the outer edge of the embryonic disk 
of eggs having so much yolk that the disk is spread out over the ventral 
surface only, not being folded completely around the yolk as in the case 
of Lepas. It appears, then, that, though the mesoblast of Lepas is dor- 
sal and that of yolk-laden eggs of higher Crustacea ventral, the two 
may be regarded as having homologous positions. In comparing Lepas 
with most other Crustacea the blastopore may be considered as having 
the same relative position, and the germ-layers may be compared with 
reference to their method of formation at the blastopore and their 
extension from that region. 
Groom (94, p. 199), who regarded the mesoblast and entoblast as 
originating from a single yolk-cell after the blastopore is closed, was 
necessarily led to the conclusion that “‘ with respect to the origin of the 
mesoblast and hypoblast of the Nauplius, the cirripedes occupy an iso- 
lated position among Crustacea.’ This statement is based upon his 
view that the yolk-cells after the closing of the blastopore constitute the 
mes-entoblast. This view is at variance with the conditions in other 
Crustacea, for the mesoblast commonly originates from the blastoderm 
and not from yolk-cells lying beneath that structure. In this paper it 
has been shown that, in general terms, the mesoblast in Lepas origin- 
ates from the blastoderm, and that, consequently, Groom’s view is 
incorrect. 
The accounts of most earlier workers on cirripede embryology lead to 
conclusions practically the same as Groom’s. In opposition to such 
conclusions it will be pointed out in the following discussion that in the 
formation of the germ-layers there are many fundamental resemblances 
between Lepas and other Crustacea. 
Among all Crustacea whose embryology is at present known, the 
closest resemblance to the development of Cirripedia appears to be 
found among the Phyllopoda and Copepoda, especially the latter. In 
the preceding chapter reference has been made to similarity of cleavage 
in these three groups of Entomostraca, but here the comparison between 
the germ-layers is to be emphasized. — 
Urbanowicz (’86) has studied the germ-layers of the copepod Cyclops 
and has found only one entoblast cell, over which the ectoblast grows 
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