60 Proceedings of the Royal Physical Society. 



narrower than the lateral portions, its transverse diameter 

 not exceeding O'Oo mm. ; in other respects it presented no 

 variations woithy of notice. 



I have observed the presence of a similar organ to that 

 above described in sections of embr3^os of Loligo and O711- 

 mastrephes, but in them only the median portion was 

 observed, no trace being discoverable of the lateral portions 

 which extend over the fins. In the former genus the patch 

 of modified cells was wider than in Sepia, and seemed to 

 cover a considerable proportion of the posterior extremity of 

 the embryo, whilst in Ommastre]ohes it was reduced to a 

 mere conical plug. 



With regard to the functions and morphological value of 

 this organ but little can be said. The nature of the cells 

 seems to forbid the hypothesis that it is sensory. If, as 

 seems most likely, it be glandular, then arises the further 

 query, What are the nature and properties of its secretion ? 



With respect to its homology there are only two structures 

 known to me with which it can be compared. The first of 

 these is the shell-gland, the second the invaginated gland 

 described at the posterior extremity of Sepiella ; and it must 

 not be forgotten that these two may possibly be modifica- 

 tions of the same primitive apparatus. In this regard its 

 extension on to the fins seems remarkable if not inexplicable. 

 These questions and others which at once suggest themselves 

 must be left in abeyance till we have further information as 

 to the origin and ultimate fate of the cells above described. 



In conclusion, I desire to express my indebtedness to Mrs 

 H. L. Calder, for her kindness in making me the sections 

 upon which these observations were made. 



VI T. A Theory of the Parasitic Hahit of the Cuckoo. By 

 J. Arthur Thomson, Esq., M.A., F.E.S.E. 



(Read 19th December 1888.) 



The parasitic habit of the cuckoo has for long been a 

 familiar subject of popular interest and of scientific inquiry. 

 Many mistaken observations and fallacious theories in regard 



