1904.] NATURAL SCIENCES OF PHILADELPHIA. 333 



this distinction be dra,vm, for until that time all quadrants appear 

 identical, though doubtless cytoplasmic and nuclear differentiation is 

 present. As a result of this similarity of all the quadrants the figures, 

 until the appearance of the mesentoderm cell, have of necessity been 

 labelled arbitrarily. Of course, even in the two-cell stage lateral may 

 be distinguished from terminal areas, for by following succeeding 

 cleavages and marking the relation which the lower polar furrow bears 

 to the first cleavage plane and the later relation of both to the median 

 plane of the embryo, it can be determined that the first cleavage plane 

 is obliquely transverse to the median plane. But not until a later 

 period does posterior become distinguishable from anterior end. 



In the formation by first cleavage of two cells of equal size, Fiona 

 agrees with a large number of Mollusks and Annelids, among the former 

 of which may be mentioned Ischnochiton (Heath, 1899), Neritina 

 (Blochmann, 1881), Crepidula (Conkhn, 1897), Ercolania (Trinchese, 

 1880), Tethys (Viguier, 1898), Planorhis (Rabl, 1879, and Holmes, 

 1900), Limax (Kofoid, 1895, and Meissenheimer, 1896), and among the 

 latter Lepidonotus (Mead, 1897) and Podarke (Treadwell, 1901). 



Unequal cleavage appears to occur as commonly as equal among 

 Opisthobranchs, examples of which are Acera (Langerhans. 1873), 

 yl pZ^sia (Blochmann, 1883; Carazzi, 1900), Umbrella (Heymons, 1893) 

 afid Philine (Guiart, 1901). 



Second Cleavage. 



The second cleavage results in four cells of approximately equal 

 size. The spindles which precede it lie at right angles to the first 

 cleavage spindle, and nearly parallel to each other, the left end of each, 

 however, being slightly higher than the right, showing the Iseotrophic 

 character of the division. As cleavage proceeds this tendency becomes 

 more marked, the upper or left-hand cells (A and C) lying higher than 

 the right (B and D) . In consequence of this the second cleavage planes 

 do not meet in a line at the vegetative pole, but a portion of the original 

 first cleavage plane unites them in the ventral polar furrow ("Quer- 

 furche" or "Brechimgslinie"), the cells B and D being in contact below, 

 while A and C never meet at the lower pole. At the upper pole no fur- 

 row is present in Fiona, the four cells all joining in a common central 

 point. As is the rule among Annelids and Mollusks in which the 

 second cleavage is laeotropic, the ventral polar furrow taken in connec- 

 tion with the first cleavage plane, bends to the right when viewed from 

 the animal pole, and, vice versa, it turns to the left if considered as a 

 part of the second cleavage plane. Fiona is no exception to the above 



