li)07.] XATUHAL SCIENCES OF PHILADELI'IIIA. 325 



least as far as the oG-cell stage, arc idciitic-ally like those of Crcpidula 

 save for the enormous size of the macroiiicros. In the positions of 

 the mitotic spindles, the direction and rhythm of division, and the 

 shapes, relative sizes and ninnber of the micromeres these two genera 

 are practically identical. In the later development of Fw/^ur the yolk 

 cells do not divide, although their nuclei do, and this may be an ap- 

 proach to the condition found in meroblastic eggs. Many character- 

 istic differences appear l)etween Fidgur and Crepichda in the later 

 cleavages, and this fact is another evidence in favor of the view, 

 which I have maintained elsewhere (Conklin, 1897, 1898), that the form 

 of the early cleavage is more constant than that of the later cleavage; 

 indeed the early cleavage pattern may be reckoned as one of the most 

 conservati^'e features in the development of any gasteropod. 



The first cleavage divides the egg into two equal cells, one of \vhich 

 forms the anterior half of the future animal, the other the posterior 

 half (fig. 2). At the close of this cleavage the nuclei and cytoplasmic 

 areas rotate first in a dexiotropic and then in a laeotropic direction, as 

 in Crepidida, showing that the first cleavage is spiral in character. 



During the first cleavage and again during the second a small lobe 

 is formed at the vegetal pole, which again fuses with one of the macro- 

 meres at the close of the cleavage. This is the yolk lobe which has 

 been observed in so many different animals. In forms in which the 

 first two cleavages are equal this lobe is small, e.g., in Crepidida it is 

 quite inconspicuous, in Fidgur it is larger but still not more than one- 

 sixth the diameter of the macromere; in forms in which the first two 

 cleavages are unequal this lobe may be larger than the macromeres, 

 e.g., Urosalpin.v. McMurrich mistook this lobe for the polar body, 

 but afterwards (1896) corrected this mistake. 



The second cleavage is at right angles to the first and divides the 

 egg into right and left hah'es. It is not possible to affirm that the 

 median plane of the future animal coincides precisely with the second 

 cleavage plane, but it is evident that the two are very nearly, if not 

 exactly, coincident. In this respect, as also in the laeotropic position of 

 the spindles and the relative positions of the daughter nuclei and 

 spheres, this cleavage is similar to the corresponding one of Crcpidula. 



It is a surprising fact that a mitotic figure so small as that shown in 

 fig. 1 can bring about the division of so large a cell. It does not seem 

 possible that the mechanical influence of the amphiaster could cause so 

 large a result, except as it may serve as a stimulus to other forces. It 

 is doubtful too whether cytojilasmic movements of a vortical nature 

 are sufficient to explain the division of a cell body in which there is so 

 22 



