APPENDIX. 



Further Observations on the Cleavage and Gastrula- 



TION OF PoLYCH.ETA. 



Since the foregoing paper was sent to press I have made 

 at Naples an extensive series of observations on the early- 

 development of Polymnia nebulosa, Mont. {Tevebella Meckelii, 

 D. Ch.), Spio fuliginosus, Clp., and a form that is almost cer- 

 tainly the Aricia fcetida of Claparede. As the results cannot be 

 published in full for some time, and are an important confirma- 

 tion and extension of the work on Nereis, I may briefly review 

 them here. 



These three forms not only belong to three widely divergent 

 families of Polychaeta, and represent both Errantia and Seden- 

 taria, but in addition to this differ from Nereis in all the con- 

 ditions of embryonic development. In all three the eggs are 

 opaque, are deposited in a jelly-mass, and in Polymnia and 

 Aricia fcetida the trochophore is partially suppressed, the free- 

 swimming life being of very brief duration. In all, the four 

 macromeres continue to divide (as in Rhynchelmis) after the 

 separation of the ectoblast and mesoblast ; and in Polymnia and 

 Aricia fcetida (probably also in Spio) there is a well-marked 

 embolic gastrulation, which leads to the formation of a large 

 blastopore. 



It might, therefore, have been expected that the cleavage 

 would differ widely from that of Nereis. As a matter of fact, 

 on the contrary, it is not only quite of the same type in the 

 three forms, but shows step by step an extraordinarily detailed 

 and striking likeness to the Nereis cleavage, which may, there- 

 fore, probably be taken as typical, in all of its leading features, 

 of an extensive series of annelids. The differences are in fact 

 quite insignificant up to a period when the germ-layers have 

 become fully differentiated and the somatoblasts have assumed 

 their typical position. 

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