BIGELOW: EARLY DEVELOPMENT OF LEPAS. Ill 



cells remain undivided. The " resting " stage following the sixth cleav- 

 age normally consists of sixty-two cells. 



By the extension of the blastoderm during the sixth cleavage the blas- 

 topore is usually closed. As to the method of closing the blastopore, 

 this account completely disagrees with Groom ('94 ; see also review of 

 literature on the closing of the blastopore). 



During this cleavage the two primary mesoblasts sink beneath the 

 blastoderm as it closes over the blastopore. 



Four blastoderm cells, derived from cells a 8 , b 9 and c 8 (the first and 

 the second micromeres, ab 2 and c 8 ), are divided parallel with the surface, 

 thus cutting off four cells which lie in the yolk beneath the blastoderm. 

 These are designated " secondary mesoblasts." 



The mesoblast is, then, derived from each of the four quadrants of the 

 four-cell stage. In the cells a 8 , b z and c 3 there is mesoblast in connec- 

 tion with ectoblast (ectoblastic mesoblast), whereas in the d quadrant 

 the mesoblast arises directly from entoblast, and may be designated 

 entoblastic mesoblast. The origin of the mesoblast in Cirripedia has not 

 heretofore been traced accurately (see review of the literature on the 

 germ-layers). 



All cells sharing in the formation of the lip of the blastopore in the 

 thirty-two-cell stage, as represented in Figure 51, contribute to the 

 mesoblast. 



The blastoderm is composed of derivatives of three, and only three, 

 micromeres (6 2 , c 3 , d*- 2 ), even when the size of the yolk-mass does not 

 permit of the blastopore being closed until the following cleavage. 



10. SEVENTH CLEAVAGE. THE MESOBLAST. 



The sixty-two-cell stage has been described as embracing fifty-two 

 ectoblastic cells composing the blastoderm, which has usually grown 

 over the blastopore ; eight mesoblast cells, of which four have been 

 designated as " secondary" ; and two entoblast cells, resulting from the 

 division of the yolk-macromere. All these, excepting the two entoblast 

 cells, divide more or less synchronously and form a stage which may be 

 estimated to consist of about one hundred and twenty-two cells. The 

 planes of cleavage appear in most cases to be perpendicular to those of 

 the sixth cleavage. For convenience in description this may be desig- 

 nated the seventh cleavage. 



Figures 78-80 (Plate 9) represent a series of parasagittal sections 

 through an egg of the 122-cell stage, but some of the cells have not 

 completed the seventh cleavage. Figures 81-56 represent a series of 



