104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the cells could not have been harmonized with the general plan indi- 

 cated by the direction of the spindles of the lifth cleavage as represented 

 in Figures 44-47. 



Summary of Fifth Cleavage. 



The blastoderm cells of the sixteen-cell stage divide synchronously. 

 The primary mesoblast {d^'"^) and yolk-entoblast {d^-^) are greatly 

 delayed in cleavage. 



The blastoderm has extended far over the yolk-entoblast. 



Regular arrangement of cells of definite origin is as characteristic of 

 this as of preceding stages. 



9. Sixth Cleavage. Sixty-two Cells. Closing of the Blastopore. 



The Germ-Layers. 



The twenty-eight cells of the blastoderm of the thirty-two-cell stage 

 are the first ones to undergo the sixth cleavage. Cases are often seen 

 in which all of the blastoderm cells have spiiidles arranged approxi- 

 mately perpendicular to those of the preceding cleavage. About the 

 time that the resulting fifty-six cells pass into the " resting " phase the 

 two daughter cells of the primary mesoblast (c^^ ■^ c^®"*) are found to be 

 in division. The two entoblast nuclei (t/'*, d^-^) remain undivided 

 until a much later stage. The sixth cleavage, therefore, results in the 

 formation of a sixty-two-cell stage. 



A preliminary description of the sixty-two-cell stage resulting from 

 the sixth cleavage will aid in the discussion of the details of that 

 cleavage. Figure 5^ (Plate 7) represents an optical sagittal section of 

 an egg with closed blastopore. All of the twenty-eight blastoderm 

 cells of the preceding stage have divided. The two yolk-entoblasts 

 ((/''•*, d^'"^) have not divided. The two mesoblast cells (t^**, (/'•*) are in 

 the sixth cleavage. Two cells {b'''^ and c''^) are represented between 

 these mesoblasts and the blastoderm in the region of the closed blastopore. 

 These two cells contribute to the mesoblast of the embryo, and for pur- 

 poses of description they may be called the "secondary mesoblasts," to 

 distinguish them from the mesoblasts, fZ*-"and </'•*, which are derived from 

 the primary mesoblast d^'"^ (Plate 5, Figs. 44, 45), which was separated 

 from the yolk-entoblast in the fourth cleavage. Referring to Figures 72 

 and 73 (Plate 8), which represent transverse sections, it will be seen that 

 there are two pairs of "secondary mesoblasts" (ms'bl'.), an anterior pair, 

 6'''*and b^-'' (compare Plate 7, Fig. 62), and a posterior pair, a'** and c'-^. 

 The series of sections represented by Figures 74-77 (Plate 9) shows con- 



