BIGELOW: EARLY DEVELOPMENT OF LEPAS. 119 



in the yolk-cell, in the sixteen-cell stage, and the separation of a 

 mesobiast cell is now known to have been an erroneous interpretation. 

 The series of stages is now so complete as to leave no doubt that the 

 mesobiast cell is separated from the yolk-entoblast in the fourth and 

 not in the fifth cleavage. 



In the sixty-two-cell stage the origin and position of cells is certainly 

 the same as in L. anatifera. The " secondary mesoblasts" were observed 

 and figured during my earlier studies, but were interpreted as deriva- 

 tives of the primary mesobiast, which seemed to divide more rapidly 

 than did the other cells. It now appears from a study of all phases of 

 the sixth cleavage that there are eight mesobiast cells in the sixty-two- 

 cell stage, only four of which are derived directly from the ectoblast. 

 Up to this stage the divisions of the primary mesobiast are the same 

 as have been described in detail in the case of L. anatifera. In living 

 eggs recently studied, and also in preparations of favorably preserved 

 material, I have observed the cell- wall between the two entoblast 

 nuclei of this stage, and it follows that contrary to my former sup- 

 position there is no exception to the rule that every nuclear division 

 during the cleavage is associated with total cell division. 



VIII. Extension of the Mesobiast and Entoblast. Later 

 Development of the Germ-Layers. 



The mesobiast in the 122-cell stage consists of a mass of cells at the 

 posterior end of the embryo, near the former position of the blastopore 

 (Plate 9, Figs. 78-86). The arrangement of the cells leaves no doubt 

 about the position of the blastopore, but orientation of the succeeding 

 stage is more difficult and uncertain. During the next division the 

 embryo begins to elongate posteriorly. A comparison of the blastoderm 

 cells on the ventral surface of the 122-cell and 250-cell (estimated num- 

 bers) stages leads to the suggestion that the elongation is due to flat- 

 tening of the ventral blastoderm cells, while those on the dorsal surface 

 remain columnar in form. At any rate, this elongation appears to be 

 confined mostly to the ventral region of the blastoderm, anterior to the 

 former position of the blastopore. The result is that the cells which 

 closed the blastopore and the adjoining mesobiast cells are moved from 

 the ventral surface towards the extreme posterior end, where for a time 

 the mesobiast consists of a conical mass of cells (compare Plate 9, Fig. 

 80 with Plate 10, Fig. 87). The rapid division of the mesobiast cells 

 produces a plate, which grows forward on the dorsal side of the embryo 



