

6 4 Miscella neons . 



gracilis, S. ManUlli, 8. Phillipsii, S. Silliniani, cfec, I have been 

 fortunate enough to discover the following North -German forms: — 



Pecopteris Geinitzii, 



Pecopteris Murchisoni, 



Pterophyllum schauinburgense (Dunker). 



and an undetermined one, which I think is Sphenopteris Gcepperti. 

 They all occur in the beds of stone in the Wadhurst Clay, which are 

 locally used for building and road-metal. 



MISCELLANEOUS. 



On the Significance of the Polar Cells of Insects. 



By M. Balbiani. 



There is now scarcely any one who admits the homology of the 

 polar cells of insects with the bodies designated by the same name, 

 or more frequently by that of direction-vesicles, in animals of other 

 classes, especially the Mollusca and Vermes. Notwithstanding 

 their extreme resemblance, it is well known that a capital diffe- 

 rence exists between these two kinds of elements : the direction- 

 vesicles disappear without taking any part in the formation of the 

 embryo, while the polar cells persist and penetrate into the ovum in 

 course of development. But authors are not agreed as to the part 

 played by these elements in the phenomena of organogeny. The 

 first observers, MM. Robin (1862) and Weismann (1863), supposed 

 that they penetrated into the blastoderm to become confounded with 

 the cells of that membrane ; but they could not ascertain what 

 became of them in the subsequent evolution, Alex. Brandt, in 

 1878, was no more fortunate than his predecessors. Metschnikoff, 

 in 1866, studying the development of the viviparous larvae of Ceci- 

 domyids (Miastor), was led to see in the polar cells the rudiments 

 of the organ in which is produced the living progeny by which 

 these Diptera multiply during a great part of their existence. But 

 this observation of the Kussian embryologist has remained com- 

 pletely isolated ; and moreover the singularity of the phenomena 

 of reproduction in Miastor did not authorize the extension of his 

 conclusions to the other animals of the same class. Consequently 

 the significance of the polar cells has remained in much obscurity, 

 and the last author who has paid attention to the question (Weis- 

 mann) could say in a recent memoir (1882) that there is no reason 

 for modifying the name under which these bodies are known so long 

 as the part they perform in the formation of the embryo is not placed 

 above all uncertainty* 



In an insect reproducing by the normal mode of fecundated and 

 deposited ova (Chironomus) I have succeeded in tracing the trans- 

 formations of the polar cells in the whole series of phases of embryo- 

 nic development, from the moment of their first appearance up to 



! 



