530 A. M. GAKK SAIINDKHS AND MARGARET POOLK. 



seem to involve serious difficulties us to the homology of the 

 orgaus formed by these very different processes, but it would 

 be clearly absurd to argue from this want of resemblauce in 

 the method by which the cells giving rise to the coelom are 

 segregated during development that the ccelom and its 

 derivatives are therefore not homologous throughout the 

 MoUusca. Evidently the heart, pericardium, and kidneys of 

 adult molluscs are all homologous. It might thus seem that 

 the evidence of embryology was worthless in this case ; but 

 these two modes of development are not so different as might 

 seem at first sight. For, although a superficial examination 

 of Molluscan cell lineages leads one to expect that meso- 

 dermal structures are always formed from the descendants of 

 4(Zat a parallel stage in development, closer inspection shows 

 that this is by no means invariably the case. The period at 

 which the mesoderm becomes segregated from the other 

 embryonic elements varies considerably ; it takes place in 

 P I a n o r b i s ra a r g i n a t u s when there are only twenty-four cells 

 present; in Planorbis trivolvis when there are forty-nine ; 

 and in Trochus magn us when there are 145, Statements 

 have also been made that in Tethys and Tei'edo 4d does not 

 give rise to mesoderm at all. Differences in the mode of 

 segregation are thus to be found in closely allied genera, and 

 we cannot lay down any hard and fast rule to govern 

 developmental processes even in the same phylum. All that 

 we are justified in saying in the present state of our know- 

 ledge is that there are certain definite organ-forming 

 substances present in the egg before segmentation begins 

 which are homologous throughout the group. As this process 

 takes place these may be separated into definite cells or groups 

 of cells, from which the corresponding organs, or complex of 

 organs, are subsequentlv developed ; but this is by no means 

 necessarily the case. The factors for the formation of 

 certain organs, as, for example, the ccelom and related struc- 

 tures above mentioned, instead of being aggregated at an 

 early stage into a single cell, may be localised, in many 

 different cells with a totally different destiny, and only at a 



