No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 437 



the period of fertilization and the occurrence of the first divi- 

 sion of the ovum. The direction of the first cleavage plane and 

 the direction of the spiral tendency of the first cleavage (whether 

 to the right or the left) are two different things and may well 

 be determined at different times. Roux has discovered that, in 

 the frog's ^g^, the plane of inclination of the Qgg axis, as well 

 as the direction of the first cleavage, is determined by the en- 

 trance path of the spermatozoon. Although in Rana cscidetita 

 the axis of the eggs is oblique before fertilization, it was found 

 by Roux that the inclination of the primitive &gg axis stands in 

 no constant relation to inclination of the axis of the fertilized 

 t.g%, which is determined by the entrance of the sperm. There 

 is thus in the frog's egg a redistribution of some of the ^gg 

 substances after fertilization and before the first cleavage, with 

 reference to the future plane of symmetry, if there be not a 

 differentiation also of the cytoplasm. That the period between 

 fertilization and cleavage is a period of active differentiation in 

 the eggs of many forms is a conclusion to which many facts 

 point. Further investigations will have to be made, however, 

 before it can with certainty be determined whether, in eggs 

 with spiral cleavage, the chief axes are established before the 

 first division of the ovum by any sort of differentiation of the 

 Qgg substance. 



In eggs with bilateral cleavage it is the rule that the first 

 cleavage plane and the median axis of the embryo coincide. 

 There are several exceptions to this rule, however, but they 

 occur in forms in which the cleavage is not of a highly deter- 

 minate type, as is shown by the variations in the cleavage of 

 different eggs of the same form. Where the cleavage is defi- 

 nite and determinate, the cell divisions occurring with regularity 

 and precision, with little individual variation in the eggs of the 

 same species, the direction of the first cleavage plane, in eggs 

 with bilateral cleavage, appears to mark accurately in almost 

 every case the direction of the median axis of the embryo. 

 This is notably the case in the cleavage of the ctenophores 

 (Metschnikoff, Agassiz, Chun). In the cephalopods bilateral 

 cleavage is again beautifully illustrated (Kolliker, Bobretzky, 

 Vialleton, Ussow, '81; Watas^, '91); though subject to some 



