122 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the longitudinal furrow laterally and ventrally is shown in Figure 94, 

 which also shows the cctoblast and mcsoblast composing the appendages. 

 The deepening of tlic furrows progresses and the appendages are folded 

 off commencing at their dorsal distal ends until linally their attachment 

 is to the ventral side of the embx'yo, as determined by the position of the 

 mouth and labrum (Figs. 124, 126). It will be seen that my account 

 contirras Groom in that the mesoblast band and the furrows are dorsal, 

 and that the appendages are fo*lded 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 descriptions and 

 figures of the formation of apj)endages, 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Many of my preparations and unpublished figures of later stages con- 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 stomodseum and 

 proctodseum, 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 sunmaarized as follows : — The ectoblast forms the outer 

 covering of the body and appendages, the stomodasum, proctodicum, 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 vind Heider ('90-91) have classed the cleavage of the cirri- 

 pede ovum with their typfe II of crustacean cleavage — a type beginning 

 with total cleavage, but soon changing to superficial. This classification 



