174 CONK LIN. [Vol. XIII. 



last-named classes, viz., that having reference to the symmetry 

 of cleavage and that having reference to its prospective value 

 in the developing organism. 



(a) The Radial Type. 



(i) Orthoradial Cleavage. — The purely radial form of cleav- 

 age, in which a series of "meridional" furrows alternate with 

 a series of "equatorial" ones, and in which the cleavage cells 

 as well as furrows form zones and meridians which are perpen- 

 dicular the one to the other, has long been regarded as the 

 typical form of all total cleavage. Thus, in the earlier works 

 on the cleavage in the frog, Amphioxus, mollusks, echino- 

 derms, in fact most animals, it is usually stated that there is a 

 regular alternation of meridional and equatorial furrows, and 

 that the number of cells is doubled at every stage ; accord- 

 ingly it was once supposed that the entire number of cells 

 could be accurately determined by getting the approximate 

 number of nuclei present on one side of the ^gg. And even 

 to this day there are those who lightly speak of 64-, 128-, 

 256-, and 512-cell stages in a way that causes one who has 

 ever attempted a detailed study of cleavage, especially in 

 these later stages, to stand aghast. Rauber ('82) has rendered 

 great service by pointing out the fact that in the frog's egg all 

 such representations are pure diagrams which have no counter- 

 part in nature. Nevertheless these ideas of cleavage have 

 found secure and undisputed lodgment in many of the text- 

 books, and so the error is propagated from year to year, and 

 from generation to generation. All such ideas of a purely 

 radial, or orthoradial,^ type of cleavage with the cells regularly 

 increasing in geometrical ratio are misleading, if not absolutely 

 erroneous. Instead of being the usual form of cleavage, this 

 orthoradial type is exceedingly rare and is never found beyond 



1 Wilson designates as a " purely or truly radial " form of cleavage that in 

 which " there are two systems of cleavage planes, of which one set are meridional 

 and radially symmetrical to the egg axis, while the other set intersect the merid- 

 ians at right angles." I propose for this form of cleavage the term Orthoradial, 

 since the so-called "spiral" form of cleavage is just as truly and purely radial as 

 the other. 



