No. I.] THE EMBRYOLOGY OF CREPIDULA. 165 



leads to the view that in those animals with very definite forms 

 of cleavage the position of the furrows and of the cells is 

 causally related to the future axes and organs. 



I think it may fairly be doubted whether in any case of spiral 

 cleavage the first furrow is bilateral with regard to the first two 

 blastomeres, even if it is bilateral with regard to the future 

 animal. In Crepidula the first cleavage is radial and the two 

 blastomeres are congruent, not bilateral, antimeres, as is shown 

 by the position of the nuclei and protoplasmic fields subsequent 

 to division. Fig. 6. In all cases the second cleavage is radial, 

 as is shown by the position of the nuclei, and above all by the 

 presence of a polar furrow. Hence, neither of the first two 

 furrows is bilateral with reference to the blastomeres, and there- 

 fore neither could be strictly bilateral with reference to the 

 adult. In fact, not a single bilateral cleavage occurs until after 

 the formation of the mesoblast (the cell 4d), and consequently 

 the egg could never, either before or after the formation of 4d, 

 be divided into bilaterally symmetrical halves along the planes 

 of one or any number of cleavages. And yet in many eggs, as 

 has been indicated, the position of the future plane of bilateral 

 symmetry can be determined approximately even as early as the 

 first or second cleavage, e.g., Planorbis, Teredo, Nereis, etc. 

 Such determination, however, is only approximate, since in later 

 stages certain of the micromeres shift across the line of symmetry 

 from one side to the other. 



That the axial relations of the first two cleavages can have 

 no such general significance as was once supposed is beyond 

 question. That they may have, however, very constant and 

 definite relations within or between certain groups of animals, 

 in particular annelids, lamellibranchs, and gasteropods, is the 

 thesis here maintained. At first thought there seem to be 

 many observations opposed even to this limited and modified 

 form of the old doctrine. In fact, among the Mollusca alone 

 three different relations of the first cleavage to the median 

 axis of the embryo are known to exist : (i) it may coincide 

 with that axis, e.g., Loligo ; (2) it may lie at right angles to it, 

 e.g.. Teredo, Crepidula, Umbrella; (3) it may cross it at a defi- 

 nite oblique angle, e.g., Planorbis, Neritina, Aplysia. The 



