1904.] NATURAL SCIENCES OF PHILADELPHIA. 337 
unaccountable way, become confused with regard to the direction of 
cleavage of these cells, for in another place, after quoting Conklin’s 
statement regarding the trochoblasts of Crepidula, that these cells 
“continue to rotate in a clockwise direction,” he adds ‘‘E la sua fig. 16 
mostra i fusi dessiotropic”. As any one acquainted with cell-lineage 
work can see by reference to the figure mentioned, the upper ends of 
the spindles all lie to the left of the lower, and if there be any question 
as to the ultimate leotropic direction of these cleavages a glance at 
Conklin’s fig. 17 removes all doubt. In T’rochus (Robert, 1903), Crepi- 
dula (Conklin, 1897) and Fiona the trochoblasts are given off by divi- 
sion of the four cells of the first quartet before the second quartet cells 
divide. In the case of Trochus the second quartet is Just being formed 
when the trochoblasts divide. Moreover, Trochus shows no rest stage 
at twenty-four cells as do the other two, for while the third quartet is 
forming and the second is dividing for the first time all eight cells of 
the first quartet again divide, and these cleavages are followed by re- 
newed division of second quartet cells. The mesoblast cell, 4d, does 
not form in TJ’rochus at this time but much later (sixty-four-cell stage), 
while in Crepidula and Fiona it appears immediately after a short rest 
period following the twenty-four-cell stage. The sequence of cleavages 
of Planorbis (Holmes, 1900) up to the twenty-four-cell stage closely 
follows Crepidula and Fiona. 
Segregation of Ento-Mesoblast. 
After a period of rest during which no cells are dividing and twenty- 
four are present in the egg, cleavage occurs in one of the macromeres. 
This macromere corresponds to that which has heretofore been arbi- 
trarily designated 3D, and from this period onward the four quartets 
may be definitely distinguished. The division is leotropie and the 
larger daughter cell, 4d, will later gradually sink into the segmentation 
cavity, forming a depression at the posterior end of the vegetative 
surface in the angle formed by the macromeres 3C and 4D, and other- 
wise bounded by 3d, 3c and the derivatives of 2d. 4d is thrown toward 
the left and, therefore, in the direction of the median plane, though at 
first it does not lie quite in that plane but slightly to the left of it or, 
in terms of spiral cleavage, to its right (Pl. XXIV, fig. 24). In con- 
tradistinction to conditions found in heavily yolk-ladened eggs, this 
cell takes on from the beginning the position of a middle germ layer 
coming shortly to lie within the cleavage cavity, though, as will be seen 
later, its derivatives do not all appear to be mesodermal in’character. 
After all three quartets and also the macromeres with the exception 
22 
