BIGELOW: EARLY DEVELOPMENT OF LEPAS. 91 



the arrangement of the cells in the four-cell stage and of the spindles 

 for the next cleavage are such that the daughter cells invariably assume 

 definite and constant positions in the eight-cell stage. 



Summary of the Second Cleavage. 



Both cells of the two-cell stage divide nearly or quite simultaneously. 

 The second cleavage plane is meridional and perpendicular to that of the 

 first cleavage. The first micromere (ab 2 ) divides equally, whereas the 

 yolk-cell cd 2 divides unequally, giving rise to the second micromere, e 3 . 



After the second cleavage the four cells (a 8 , b*, c 3 , c? 3 ) become adjusted 

 in a Iseotropic arrangement. 



In the four-cell stage a plane passing through the second polar cell 

 and the nuclei of cells b* and d? is apparently near the- sagittal plane of 

 the future embryo. In this stage, then, there is a suggestion of bilateral 

 arrangement of the cells. 



The yolk-cell undergoes ordinary unequal cleavage (see the following 

 review of the literature). 



5. EEVIEW OF LITERATURE ox SECOND AND SUCCEEDING CLEAVAGES. 



In this connection it is necessary to give a general review of the litera- 

 ture bearing on all early cleavages after the first, because no previous 

 worker has recognized definite stages into which the cleavages of the 

 cirripede ovum can be grouped. It is therefore impossible to make any 

 comparison of my account with that of others, except in a general way. 



The division of the " protoplasmic " cell (ab 2 ) of the two-cell stage of 

 the cirripede egg has been correctly described by most authors. The 

 plane of cleavage has been generally described as perpendicular to the 

 first cleavage plane, but Nussbaum (*90) has recognized that in Polli- 

 cipes it intersects the first cleavage plane at the polar cell and is, there- 

 fore, meridional. 



No investigator of the early development of Cirripedia, except Groom, 

 has shown that the yolk-cell, cd 2 , of the two-cell stage divides and adds 

 new cells to the blastoderm. All other observers, Buchholz ('69), Hoek 

 ('76), Lang ('78), Nassonow ('87), and Nussbaum ('90), have described 

 the yolk-cell cd 2 as remaining undivided while the other cell (ab 2 ) re- 

 peatedly divides and its products grow around the yolk-cell, forming the 

 blastoderm. After completion of the blastoderm, and closing of the 

 blastopore, the yolk-cell cd 2 was said to divide, separating the mesoblast 

 from the entoblast. According to this view the cell ab 2 , which forms 



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