No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 291 



plate is composed of four cells : in other forms three of these 

 divide on the surface and give rise to an entoderm plate of seven 

 cells. This appears to be the case in the molluscs, Planorbis, 

 Neritina, and Umbrella. In LepidonoUis the same number of 

 divisions obtains, but we do not know how many or what cells 

 are mesodermal. In several annelids, Polymnia, Aricia, Amphi- 

 irite, Clymenella, the four inner cells of the plate divide again, 

 making the entoderm plate number eleven cells. 



The ova of polychastous annelids, and of many other forms 

 in related phyla pass through cleavage stages easily referable to 

 a common type; the regularity and topographical peculiarities 

 of these stages have made it possible to compare the corre- 

 sponding cells among the various animals in all stages of cleav- 

 age, while the very early differentiations of the blastomeres 

 often makes this comparison of paramount theoretical value. 

 The more extensive and thorough the comparison, the better 

 warranted the inference that there exists between the cleavage 

 cells of various forms the same morphological relationship 

 which exists between adult organs and tissues, i.e., homology. 



The main objections urged against cell homology in this type 

 of cleavage are the alleged difference in the fate of the cell a"'' in 

 Nereis and in Unio, and the difference in origin of the mesoblast 

 in Discocalis and in the other forms. On closer scrutiny, these 

 objections lose their force because of a seeming oversight in 

 the first instance, and the lack of evidence in the second. 



In attempting to explain annelid cleavage by means of the 

 mechanical principles already deduced from experimental sources 

 and applied with varying success to other forms of cleavage, 

 two points present themselves: first, in the echinoderms, ver- 

 tebrates, etc., very little has been ascertained concerning the 

 normal relations between the cleavage cells and the organs and 

 tissues of the adult of the same individual, and between the cor- 

 responding cleavage cells of different species; second, in the 

 annelids, etc., while considerable is known concerning both of 

 these normal relations, no successful experiments have been 



