1 64 CONK LIN. [Vol. XIII. 



('83) found the same thing true of Aplysia, and his figures of 

 the segmenting egg of Neritina ('81) show that here also the 

 first two furrows are oblique to the median plane of the embryo. 

 The same seems to be true of Bobretzky's ('77) figures of 

 Fusus, and Salensky's ('87) figures of Vermetus, though in 

 neither case is this matter mentioned in the text. 



In all these gasteropods except Aplysia four nearly equal 

 macromeres are formed ; but in other cases one macromere is 

 often much larger than either of the other three. Among 

 prosobranchs and lamellibranchs this larger macromere is 

 usually in the median line behind, e.g., Nassa, Illyonassa, Uro- 

 salpinx, Tritia, Ostrea, Unio, etc., and in all such cases the 

 plane of bilateral symmetry passes through the middle of the 

 larger macromere. Among opisthobranehs and pteropods one 

 or two of the macromeres are frequently smaller than the 

 others, and these lie at the posterior side of the Qgg, e.g., 

 Aplysia, Umbrella, Cavolina, etc. In such cases the plane 

 of bilateral symmetry may or may not pass through the smallest 

 macromere. 



In still other animals it has been found that the first cleavage 

 apparently bears no definite and constant relation to the median 

 plane, e.g., in the toad fish Miss Clapp ('91) has demonstrated 

 that the axis of the embryo may lie in the direction of the first 

 cleavage furrow, or may vary as much as 70° from it; Morgan 

 ('93) asserts that in Ctenolabrus and Serranus "there is no 

 relation whatsoever between the cleavage planes of the egg 

 and the median plane of the adult body"; Jordan ('93) has 

 found in the newt that the median plane may vary more or less 

 from the plane of the first cleavage, and Jordan and Eycleshymer 

 ('94) have found the same thing true of Amblystoma. 



Such a series of discordant phenomena might well cause one 

 to regard the axial relations of the first and second cleavages as 

 of no morphological consequence whatsoever. The fact, how- 

 ever, that in many ova there is an absolutely constant relation 

 between the cleavage planes and the axes of the embryo shows 

 that in these cases the position of the cleavage planes and of the 

 resulting blastomeres is not a matter of chance; and this, taken 

 in connection with many other phenomena of similar character, 



