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 



