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 Iseotropic direction of these cleavages a glance at 

 Conklin's fig. 17 removes all doubt. In Trochus (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 cjuartet is just being formed 

 when the trochoblasts divide. ]\Ioreover, Trochus shows no rest stage 

 at twenty-four cells as do the other two, for while the third c^uartet 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 Trochus 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 Planorhis (Holmes, 1900) up to the twenty-four-cell stage closely 

 follows Crepidula and Fiona. 



Segregation of Ento-Mesohlast. 



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 Iseotropic 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 (PI. 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 



