No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 431 



mal. In most cases the first cleavage plane has been found to 

 be oblique to this axis, and in many forms the median axis is 

 found to lie approximately midway between the first two cleav- 

 age planes (Neritina, Physa, Planorbis, Clepsine, Discocelis). 

 In some instances, however, the first cleavage plane is said to 

 be transverse to the median axis of the embryo (Nereis, Um- 

 brella, Teredo). The instances in which the first cleavage 

 plane is said to be oblique are by far the most numerous, and 

 it will be well to devote some attention to the purported excep- 

 tions to this rule, with a view to determine whether these 

 exceptions are not more apparent than real. The first cleavage 

 furrow, in advanced stages of cleavage, comes to follow a very 

 crooked path, and the upper portion may run in a quite differ- 

 ent direction from the lower. Since the entomeres are usually 

 of large size, the portion of the first cleavage furrow lying be- 

 tween these cells has generally been taken to indicate the direc- 

 tion of the first plane of cleavage, while the upper part of this 

 furrow lying between the ectomeres has been disregarded. It 

 is this circumstance, as will appear, that gives rise to the dif- 

 ferent accounts of the axial relations of the first cleavage plane. 

 In the ectodermic portion of the egg the axial relations of the 

 first two cleavage planes are remarkably constant in all forms 

 zvith spiral cleavage. The different quartettes of ectomeres 

 have exactly the same relative arrangement in annelids, mol- 

 lusks, and polyclades ; and cells of the same origin in these 

 forms have almost identical axial relations. In both annelids 

 and mollusks certain cells become arranged in the form of a 

 cross, the arms of which have very constant relations to the 

 embryonic axes. The arms of the cross in mollusks, which are 

 always made up of cells of similar origin, are always anterior, pos- 

 terior, right and left. In the annelids the cross is mainly formed 

 of cells corresponding to those lying between the arms of the 

 molluscan cross, and the arms lie at an angle of about 45° with the 

 median axis of the embryo. As the upper portions of the first 

 two cleavage furrows pass between cells of corresponding origin 

 in all these forms, they maintain almost as constant axial rela- 

 tions as the arms of the cross themselves. It may be said that 

 the longitudinal axis typically bisects the angle between the tipper 



