No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 289 



However, I am not convinced that the cells described by 

 Lang do give rise to the mesoderm, and I believe it possible 

 that the mesoderm is formed in the same manner and from 

 exactly the same cell as in the annelids with unequal cleavage. 



Lang figures the cleavage up to about the 64-cell stage. 

 At this time the cells which, according to his description, 

 become the mesoderm, lie upon the surface, and even 

 extend slightly over the ectoderm. To determine the exact 

 cell lineage of any organ requires a close and uninterrupted 

 series of observations. The stages from 64 cells to the 

 end of gastrulation, are, of course, the critical stages for 

 establishing the origin of the mesoderm. Lang does not 

 record, either in text or in figures, any observations covering 

 this important period. 



We now naturally turn our attention to those cells in Disco- 

 ccelis, which in other animals, with this type of cleavage, regu- 

 larly give rise to the mesoderm bands. Lang's Fig. 12, Plate 

 XXXV, of the lower pole of Discoccelis corresponds in every 

 essential point to the ideal 32-cell stage of annelids and mol- 

 luscs with unequal cleavage. All the cells have the same 

 origin and relative position, and belong to the same generation. 

 The four vegetative cells are comparatively large, and one 

 exceeds the others in size. Now in the annelids these four cells 

 regularly divide upon the surface; the eight resulting cells are 

 of the same generation and lie in a characteristic position, four 

 meeting at the vegetative pole and four alternating with them. 

 One of the latter, the offspring of the larger cell (d quadrant), 

 is always the mesoderm cell, and lies in the middle line of the 

 ftiture body, while the other seven are entodermal. In Nereis 

 only the larger cell divides on the surface, so that the entoderm 

 has but four instead of seven cells; a fact which only empha- 

 sizes the significance of the division giving rise to the mesoderm 

 cell. In all forms where its origin is known, the mesoderm cell 

 begins at once to behave in a perfectly unique and character- 

 istic way. — It always divides into two equal cells, one of which 

 lies on the right and one on the left of the middle line. 

 These cells give rise to the right and left mesoderm bands 

 respectively. 



