BIGELOW: EARLY DEVELOPMENT OF LEPAS. 107 



they become flattened and massed together at the extreme posterior 

 end of the egg (Fig. 121). 



The amount of yolk in the entoblast cells is in some eggs so great 

 that the blastoderm cannot completely close over the blastopore during 

 the sixth cleavage. Eggs are sometimes seen in which all the blasto- 

 derm cells have undergone the sixth cleavage and the two primary 

 mesoblasts, still in division, are seen lying in the blastopore, and pro- 

 jecting far into the yolk (Plate 7, Figs. 60, 61). The anterior pair of 

 " secondary mesoblasts " (6 7 - 6 , 6 7 - 7 ) are seen in their usual place beneath 

 the blastoderm immediately in front of the anterior edge of the blasto- 

 pore ; but the posterior pair (a 7 - 5 , c 7 - 5 ), which originates from cells lying 

 at the sides of the blastopore, are seen at the surface at the side of the 

 primary mesoblasts (Fig. 60). As these primary mesoblast cells com- 

 plete the sixth cleavage they move farther into the yolk. Their posi- 

 tions with reference to the surrounding blastoderm cells (Fig. 61) 

 suggests that the change of form during cleavage results in a movement 

 of the dividing cells into the yolk, in which direction there is, appar- 

 ently, the least resistance. The posterior pair of "secondary meso- 

 blasts " (a 7 * 5 , c 7 ' 5 ) sink below the level of the surface as the blastoderm 

 closes over the blastopore. In many cases this closing is evidently 

 brought about by the next (seventh) cleavage of the blastoderm cells. 

 Certainly the blastopore is always closed and both the primary and 

 " secondary mesoblasts " are completely covered by the blastoderm 

 after the seventh cleavage. 



The origin of the two pairs of the " secondary mesoblasts " now re- 

 mains to be described. Careful study of the cleavage in numerous eggs 

 gives evidence that these are the result of the sixth cleavage in the four 

 blastoderm cells, a*- 8 , *- 8 , i 6 - 4 , c 6 - 8 , which form the lateral and anterior 

 boundaries of the blastopore in the thirty-two-cell stage (Plate 6 r Figs. 

 51, 52). These four blastoderm cells have their spindles for the sixth 

 cleavage arranged more or less perpendicular to the surface, as shown in. 

 Figures 58 and 59 (Plate 7). The anterior pair of " secondary meso- 

 blasts " (6 7 - 5 , & 7 - 7 ) lies in front of the anterior edge of the blastopore, as 

 is shown in Figure 57, which represents a section through an egg with 

 incompletely closed blastopore. This is exactly the position of the cells 

 5*- 8 and 5 6> * in the thirty-two-cell stage (Fig. 51). In Figures 58 and 59 

 (Plate 7) these cells are shown with spindles (sixth cleavage) somewhat 

 inclined from a perpendicular to the surface. Their relation to the blas- 

 topore leaves no doubt that they are the cells fi 6 ' 8 and 5 6t4 of the thirty- 

 two-cell stage. 



VOL. XL. 2 4 



