74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



microraeres are separated from the yolk-bearing raacromere. These three 

 cells contain all the ectoblast, and by their repeated division form the 

 blastoderm. Certain cells of the blastoderm, which are derived from the 

 first two micromeres, give rise to a portion of the mesoblast, hence these 

 two micromeres are not purely ectoblastic. The third contains only 

 ectoblast. In the fourth cleavage a mesoblast cell is separated from the 

 yolk-macromere, which now represents entoblast alone. 



The sixteen-cell stage, therefore, is composed of fourteen derivatives 

 of the three micromeres, one mesoblast cell, and one entoblast cell (yolk- 

 macromere). The entoblastic yolk-macromere is nearly enveloped by 

 the fourteen smaller cells composing the blastoderm, only a small part 

 of the entoblast cell being exposed at the blastopore. The single meso- 

 blast cell lies at the posterior edge of the blastopore, and were its history 

 not known would certainly be regarded as a cell of the blastoderm. At 

 the fifth cleavage each of the sixteen cells divides, the two resulting 

 mesoblastic cells still remaining at the surface. At the sixth cleavage 

 all the cells except the two entoblast cells divide, thus producing a sixty- 

 two-cell stage. During the sixth cleavage the two mesoblastic cells, 

 before dividing, sink beneath the blastoderm, as this closes over the ento- 

 blast and obliterates the blastopore. At the same time four cells of the 

 blastoderm, lying at the anterior and lateral edges of the blastopore, 

 divide parallel to the surface. The four deep cells thus formed beneath 

 the blastoderm constitute a part of the mesoblast. The mesoblast, then, 

 is derived in part from one cell which is separated from the entoblast in 

 the fourth cleavage (sixteen-cell stage) and in part from four other cells 

 which are detached from the blastoderm during the sixth cleavage. 



Gastrulation is of the epibolic type, and is the result of the extension 

 of the blastoderm over the entoblastic yolk-macromere. During the 

 sixth cleavage, which leads to the formation of a sixty-two-cell stage, the 

 blastoderm usually closes over the blastopore, which marks the ventral 

 and posterior part of the future embryo. 



In the general features of the late development of the embryo the 

 results of this investigation confirm those of some earlier workers. 



VI. Nomenclature of Cleavage. 



For convenience in describing the cell-lineage of Lepas and in making 

 comparisons with the development of other forms, it is desirable that 

 some system of cell-nomenclature should be applied. 



The common systems, which have been developed with special refer- 



