1007.] NATURAL SCIENCES (W PHILADELPHIA. ' 327 



ally (listri))uted through the yolk, into the micromeres and cytoplasmic 

 areas of the macronieres, rather than to the conversion of yolk into 

 cytoplasm, since the yolk becomes less fluid and more firm during this: 

 period . 



In Fulgur and Sycotypus, as in every other gasteropod so far studied, 

 there are only three quartets of ectomeres. This is especially inter- 

 esting in \ic\v of the enormous size of this egg, where we might expect, 

 if anywhere, to find a much larger number of micromeres formed. 

 .McMurrich supposed that a large number of micromeres were budded 

 off from the macronieres in Fulgur, and that the number of such micro- 

 meres varied with the size of the egg. Viguier (189S) and Fujita (1895) 

 also reached the same conchision, maintaining that the number of c^uar- 

 tets formed in Tethys and Siphonaria is greater than three. This view 

 is in my opinion wholly without foundation. Whether the egg be large 

 or small three cjuartcts of ectomeres are formed, no more and no less, 

 and by the subdivision of the cells of these quartets all the ectoderm 

 of the embryo is formed. In the cases cited by Yiguier and Fujita 

 it can readily be seen, from their own figures, that they have mistaken 

 the subdivision of certain quartet cells for the formation of new 

 quartets from the macronieres. 



Since there are only three quartets of micromeres in Fulgur, and since 

 these cells nuist overgrow the enormous yolk, it would seem reasonable 

 to expect that the rate of subdivision of these quartets would be 

 accelerated, as compared with forms in which the yolk is small. How- 

 ever this is not the case; the first quartet divides about the time of the 

 formation of the second quartet, and the latter divides coincidently 

 with the formation of the third quartet, just as in Crepidula. In much 

 later stages the micromeres do divide more rapidly, but the early 

 subdivisions of the three quartets is not more rapid in the case of this 

 largest of gasteropod eggs than in the case of the smallest. In every 

 respect except the size of the macromeres, the early cleavage of Fulgur 

 is almost identically like that of Crepidula or any other prosobranch. 

 In the number, position, shapes and relative sizes of the micromeres 

 and in the positions and sizes of the nuclei and spheres there is the 

 closest similarity between the largest and smallest of gasteropod eggs. 



The next division is a highly characteristic one for annelids and 

 mollusks. viz.. the formation of the first member of the fourth ([uartet, 

 4/-/, by la>otropic cleavage of the left posterior macromere. 



After its formation it moves over into the furrow between macro- 

 meres D and C, as shown in pi. XXIV, fig. 8. This cell, 4c?, is formed 

 in advance of the other members of the fourth riuartet and it contains 



