478 RECORDS 



The cleavage oi Lepas is totally unequal and regular. Stages 

 of 2, 4, 8, 1 6, 32, and 64 cells are normally formed. Cells of 

 a given generation may anticipate their fellows in division, but 

 no second division of such cells takes place before all other cells 

 have completed corresponding cleavages and become of the 

 same generation. 



The first cleavage is nearly parallel to the long axis of the 

 ellipsoidal ^%^. The ^^'g is divided into an anterior ectoblastic 

 cell and a posterior yolk-bearing macromere. The second 

 cleavage is at right angles to the first, both cells dividing, and 

 from the yolk-macromere is cut off a second ectoblastic cell. 

 The third cleavage is essentially perpendicular to the first two, 

 dividing all of the cells, and a third ectoblastic cell is separated 

 from the yolk-macromere, which is now mesentoblastic. Thus 

 by the first, second and third cleavages three cells are sepa- 

 rated from the yolk. These three cells contain all the ecto- 

 blast, and by repeated division they form the blastoderm. The 

 fourth cleavage separates the mesoblast from the ectoblast, 

 which is now represented by the yolk-macromere. 



The 1 6 -cell stage is composed of fourteen ectoblastic cells 

 which largely surround the entoblastic yolk-cell. The single 

 mesoblast cell lies in the blastoderm at the posterior edge of 

 the blastopore, where the entoblastic yolk-cell is still exposed 

 to the exterior. By the fifth cleavage all these cells are divided, 

 the two mesoblastic cells still remaining on the surface. Dur- 

 ing the sixth cleavage the two mesoblastic cells sink beneath 

 the blastoderm as it closes over the blastopore. At the same 

 time four cells of the blastoderm, lying at the anterior and lateral 

 edges of the blastopore, divide perpendicularly to the surface. 

 Four cells are thus formed, beneath the blastoderm, and they 

 are apparently added to the mesoblast. The entire mesoblast 

 then originates from one cell which is separated from the ento- 

 blast in the fourth cleav^age i6-cell stage, and from four other 

 cells which are derived from the ectoblast in the sixth cleavage 

 forming the 62-cell stage. 



The course of the cleavage as sketched above has been de- 

 termined to be quite constant. Cells of definite origin in the 



