BIGELOW : EARLY DEVELOPMENT OF LEPAS. 127 



In certain respects the cell-lineage of Lepas recalls that of some roti- 

 fers, as described by Zeliuka ('91) and especially by Jennings ('96). 

 In the rotifers, as in Lepas, the separation of the primary germ-layers 

 begins with the first cleavage, the cell ab^ being ectoblastic, and cd? 

 containing ectoblast in addition to all the entoblast. Still more remark- 

 able is the resemblance in that the entoblast is derived from the cell 

 d?'^ both in Asplanchna and in Lepas. This cell is purely entoblastic in 

 Lepas, and probably so in Asplanchna; its two minute derivatives d^-"^ 

 and d"^-"^ are regarded by Jennings as belonging to this germ-layer. The 

 macromere d^ in this rotifer, as in Lepas, gives rise to d^'"^ in the third 

 cleavage and d^'^ in the fourth. In both d*-' is purely ectoblastic. In 

 Lepas d^'"^ is the primary mesoblast, but in Asplanchna it is ectoblast. 

 However, the exact origin of the mesoblast in the rotifers is unknown. 

 It is evident that the number and order of cleavages which are involved 

 in the segregation of the entoblast from the ectoblast are the same in 

 the rotifer as in the cirripede. 



XI. Comparisons of the Germ-Layers of Lepas with those 



of other Crustacea. 



The account here given of the development of Lepas agrees with the 

 published descriptions of the development of the majority of Crustacea, 

 in that the blastopore is posterior and ventral, and apparently near the 

 position of the future anal aperture. This similarity in the relation of 

 the blastopore appears at first to be without significance, if one com- 

 pares the embryo of Lepas, which has the mesoblastic band on its dorsal 

 side, with crustacean embryos containing much yolk and having the 

 mesoblastic plate ventral in position, as it is in decapods. However, 

 the facts appear to allow of the following interpretations : In crustacean 

 eggs which are heavily laden with yolk, the embryonic disk is at first 

 confined to the ventral surface, but gradually extends dorsally over the 

 yolk-mass. The mesoblast is formed while the embryonic disk is ven- 

 tral. In Lepas, and some other Crustacea in which there is a relatively 

 small amount of yolk, the embryonic disk is not confined to the ventral 

 surface, but from the close of cleavage it is extensive enough to sur- 

 round the yolk completely. In consequence of this the mesoblast, 

 which in higher Crustacea forms bands on either side of the median 

 ventral line, in Lepas extends along the dorsal line. If one imagines an 

 ordinary decapod egg deprived of the greater part of its yolk until, at 

 the close of cleavage, the edges of the embryonic disk meet on the dor- 



