BIGELOW: EARLY DEVELOPMENT OF LEPAS. 



109 



the cell arrangement and spindles as shown in Figure 58, 60 and 62 

 gives evidence entirely in favor of the explanation given of the cell- 

 lineage of the "secondary mesoblasts." T^ey are certainly derived 

 from the ectoblastic cells of the blastoderm, and the evidence com- 

 pletely supports the interpretation that they are derived directly from 

 the cells bounding the blastopore laterally and anteriorly in the thirty- 

 two-cell stage. 



The cell-lineage of the " secondary mesoblasts " is, then, as shown in 

 the following table (see also complete table of the cell-lineage on page 

 135). 



16: 



b^-i: 



^6■S 



,6-3; 



i(7-5 right anterior "secondary mesoblast" call. 

 b~-^ blastoderm cell (ectoblast). 



b''-'' left anterior " secondary mesoblast " cell. 

 6^-8 blastoderm cell (ectoblast). 



a^-5 left posterior " secondary mesoblast " cell. 

 a'-^ blastoderm cell (ectoblast). 



c~i-5 right posterior " secondary mesoblast " cell. 

 c''S blastoderm cell (ectoblast). 



It will be noticed that " secondary mesoblast " originates from the 

 quadrants a, b, and c. One cell each is contributed by a and c but two 

 cells come from b. Tracing the lineage to the three micromeres which 

 are separated from the yolk-macromere in the first three cleavages, it is 

 found that only the first (ab^) and the second (c') contain " secondary 

 mesoblast " ; the third (d*-^) is purely ectoblastic. 



After the sixty-two-cell stage the derivatives of the " secondary meso- 

 blasts" have not been distinguished from those cells which were derived 

 from the primary mesoblast. The cells of the two origins become 

 mingled together and there appear to be in Lepas no distinguishing 

 characteristics. Hereafter the term mesoblast (nis'bl. in the figures) will 

 be used in the description as including the mesoblast cells of the two 



origms. 



The entoblast nuclei (d^-\ ^'^) are always near the primary mesoblast 

 cells, but, as shown in the figures, they occupy no constant position in 

 relation to particular cells. They stain more intensely than the nuclei 



