BIGELOW: EARLY DEVELOPMENT OF LEPAS. 105 



clusively that there are, besides the four " secondary mesoblasts," two 

 eutoblasts and two dividing primary mesoblasts in the egg of this stage. 

 The cells of the anterior pair of " secondary mesoblavSts " {b''-^, b''-'') are 

 always hemispherical in form (Fig. 73), while those of the posterior pair 

 are flattened between the primary mesoblast cells {d^-^, d^-*) and the 

 blastoderm (Fig. 72). It also appears from the figures that the two 

 derivatives of the primary mesoblast (c/^'^), the two pairs of " secondary 

 mesoblasts," and the two entoblasts, are arranged according to a plan 

 of bilateral symmetry. The division plane in the yolk (Fig. 73) is the 

 cleavage plane formed between the entoblast cells during the fifth cleav- 

 age. With this brief description of the sixty-two-cell stage we may now 

 turn to a more detailed consideration of the sixth cleavage, which formed 

 the stage. 



The large number of small cells and the absence of " landmarks " 

 makes rapid and certain identification of individual cells of the blasto- 

 derm on the dorsal surface impossible in the sixty-two-cell and later 

 stages. By carefully comparing drawings of stages in which the cells 

 of the blastoderm are in early and late stages of mitosis, it is often 

 possible to identify all the individual blastoderm cells in the sixty-two- 

 cell stage. But since it is impossible to follow the blastoderm cells 

 to their fate in organs of the Nauplius, I have not attempted to give iu 

 this account the lineage of all cells after the thirty-two-cell stage. 

 After that stage the most important cells concerned with the gei'm- 

 layers are near the blastopore. These are followed easily and with 

 certainty. 



During the fourth and fifth cleavages the blastoderm was greatly 

 extended by the flattening of its cells and by the increase of surface 

 associated with cell-division. This is repeated during the sixth cleavage, 

 and the result is that the blastoderm in the majority of cases is com- 

 pleted, the yolk-eutoblast cells being no longer exposed to the exterior 

 at the blastopore (see Plate 7, Fig. 56, and Plate 8, Fig. 71). 



In most cases a very small opening between the blastoderm cells 

 represents the remnant of the blastopore. In fact the cells bounding 

 the blastopore rarely come so closely together in this stage as to com- 

 pletely obliterate the opening (see Plate 7, Figs. 57, 60, 62 ; Plate 8, 

 Fig. 71 ; Plate 2, Fig. 76). This persistence of the blastopore has 

 been of great service in determining the origin of the " secondary mes- 

 oblasts " and in the orientation of succeeding stages. 



Along with the growth of the blastoderm over the blastopore during 



