36 BIOLOGICAL LECTURES. 



the median plane and the second coincides with it. The axial 

 relations of the first two cleavages being different in these 

 cases, it seems that the first four cells must give rise to dif- 

 ferent organs in the two classes named. However, a careful 

 examination shows that in all these cases the ectomeres and 

 mesomeres rotate so as to occupy relatively the same positions 

 and ultimately give rise to the same parts (Fig. 6) ; the position 

 of the entomeres alone is different. It seems to me very prob- 

 able, considering the extensive shifting which the entomeres 

 undergo in late stages, that even the axial differences of these 

 cells may ultimately disappear, but even if they do not it is cer- 

 tainly a matter of secondary importance that a few cells form- 

 ing a tubular internal canal should occupy slightly different 

 axial relations as compared with the fact that hundreds of 

 cells occupy relatively the same positions and give rise to the 

 same organs. The entomeres have undergone great modifica- 

 tions owing to the acquisition and loss of yolk and its varying 

 distribution to the different macromeres, and it would not be 

 surprising if they have also shifted their axial relations in some 

 cases. On the whole, this apparent difference in the axial 

 relations of the first two cleavages affords an unexpected 

 confirmation of the fundamental likeness of all gasteropod 

 cleavage. 



These important resemblances of cleavage stages are not 

 limited to the gasteropods. Wilson 1 has pointed out a number 

 of remarkable similarities in the cleavage of polyclades, anne- 

 lids, and gasteropods ; Lillie 2 has shown that the lamellibranch 

 cleavage is essentially like that of the gasteropods and annelids; 

 and Heath 3 has recently discovered that the cleavage of Chiton 

 resembles in the most wonderful manner the cleavage of all 

 the groups just named. 



" Wilson emphasizes the following important resemblances 

 between the early cleavage stages of the annelid, the polyclade, 

 and the gasteropod: (i) the number and direction of tJie cleav- 

 ages is the same in all three up to the 28-cell stage; (2) in 



1 Wilson, E. B., "The Cell Lineage of Nereis," Journal of Morphology, VI, 1892. 



2 Lillie, F. R. "The Embryology of the Unionidae,"yiwra/ of Morphology, X, 

 1895. 3 Heath's work is not yet published. 



