1904.] NATURAL SCIENXES OF PHILADELPHIA. 355 



additional landmark for distinguishing anterior from posterior quad- 

 rants (PI. XXIV, fig. 25). The larger cells of the posterior quadrants, 

 3ci and Sd^ divide [next; the spindle in Sc^ being dexiotropic and 

 alternating, that of 3d^ laeotropic and non-alternating; and this lack 

 of alternation in one of the large cells of the third quartet, taken in 

 connection with the regular alternation of the similar cell on the oppo- 

 site side of the posterior region of the egg, establishes the first bilat- 

 eral cleavage (PI. XXV, figs. 31, 32, 34). Both upper and lower cells 

 of A and B quadrants are the next third quartet elements to divide, 

 the direction in all cases being dexiotropic or in some instances nearly 

 meridional (figs. 37, 40, 41). The lower cells, 3a' and 3b-, always divide 

 before the upper, 3a^ and 3b\ and in all cases cleavage is equal, a gi'oup 

 of four similar cells arising in each of the two anterior quadrants. 

 In the posterior cpadrants cleavage occurs next in 3di-, 3d", 3c^- and 

 3c". It will be remembered that when these cells were formed it was 

 through a laeotropic and non-alternating division of 3d^ and a dexio- 

 tropic and alternating division of 3c\, thus producing a bilateral cleav- 

 age of similar cells of opposite sides. Xow the cells 3c" and 30^- 

 again divide dexiotropically, thus showing lack of alternation, while 

 3d" and 3di- again exhibit distinct Inotropic cleavage and a second 

 failure to alternate. Thus arise in each posterior quadrant two very 

 smaU cells, 3c"2, 3c^22 ^nd 3d"-, Sd^--, lying below the large ones, 3c"% 

 'Sc'-\ 3d"^ and 3cV-' (PI. XXVI, figs. 43, 44, 45, 47). After these 

 cleavages about eighty blast omeres are present (figs. 67, etc.). When 

 this number has increased to shghtly over a hundred, Ssi^\ 3a22, 3b2^ 

 and 3b2^ each gives off a small cell toward the vegetative pole by cleav- 

 ages which appear horizontal (PI. XXVII, figs. 57, 59), and these divi- 

 sions are followed by equal and probably horizontal cleavages in the 

 posterior quadrants of the large cells, 3c"^ 3d"i and 3c^-» and 3di=^\ 

 the former pair always dividing before the latter (figs. 61, 66), so that 

 each posterior group contains seven cells, of which three are small 

 and he nearest the blastopore, being bounded externally by four large 

 cells, 3c"", "^-, 1'", '^'\ and 3d"", '''-, ^-'\ '-'^ respectively. 



The history of the third quartet of Fiona thus far given adds another 

 to the niunber of Mollusks in which it has been found that bilateral 

 cleavages first appear in the posterior quadrant, and more particularly 

 in the cells of the third quartet. 



The initial divisions of these cells in UmhreUa appear from Heymons' 

 description to be nearly radial, but his figures show that in the case of 

 3c and 3d cleavage is Iseotropic. The lower products of these cleavages 

 are all smaller than the upper, in which they parallel only the posterior 



