The development of Phascolosoina. IQ]^ 



The sixteen cells in the posterior part of the embryo consist 

 of exactly the same quartets as in the thirty-two-cell stage. 



Forty-eight to Sixty-four Cells. 



My observations at Eoscoff were unavoidably broken off at a 

 time when I had followed the cleavage of the egg as a whole only 

 as far as the forty- eight-cell stage, and I have not succeeded in 

 carrying it further with preserved material, which I brought away 

 in abundance. I made there certain observations, however, upon 

 the cleavage in the succeeding stages, the most important being 

 upon the origin of the mesoderm. 



A laeotropic division of 3D results in the formation of 4d 

 (Fig. 39). This cell immediately divides at the surface in a right 

 spiral, the anterior of the two nearly equal daughter cells being 

 crowded inward, and the posterior following it, the two forming the 

 teloblasts of the mesoderm. 



The sinking of the endoderm plate immediately opposite the 

 ventral arm of the cross (Fig. D, Ih^-'^-") marks the position of the 

 future stomodaeum. This point and the dorsal interruption of the 

 prototroch (Fig. D, Id^-^-^) indicate the plane of bilateral symmetry 

 of the trochophore, which coincides approximately with a line 

 separating quadrants C and D, dorsally, and A and B, ventrally. 

 Thus the plane of bilateral symmetry corresponds roughly in the 

 anterior hemisphere to the second cleavage plane, but it actually 

 intersects the quadrants B and D, since it passes through the arms 

 of the cross that arise from these quadrants. In the posterior 

 hemisphere it approximately bisects quadrants B and D. 



6. Development of the Embryo into the Trochophore 



(10—24 hours). 



This period is characterized by the division of the rudiment 

 of the apical plate into the definitive rosette and a large number 

 of small cells, by the addition of three secondary trochoblasts to 

 the prototroch, by the growth of the somatic plate laterally and 

 ventrally by bilateral cleavage, by the growth of the mesoderm 

 bands, and by the sinking of the endoderm cells and closure of 

 the blastopore. 



Apical Plate. The divisions of the apical group of cells in 

 the latest stages of cleavage which I have studied (48 — 64 cells) 



