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larore cells of the frog's ovum at an early stage of develop- 

 ment of the blastoderm ; these large cells, having the power of 

 movement, as may be witnessed on the stage of the micro- 

 scope, advance from the floor of the cleavage cavity, and pass 

 to the roof to form part of a new layer of the blastoderm, as 

 similarly His has observed the large cells in the hen's ovum to 

 move upwards and pass between the two primitive layers of 

 the blastoderm. Again, Besiadeki has drawn attention to 

 cells occurring in the fibrous tissue of the skin, which have 

 in all probability advanced into this situation from some other 

 part, making their way as if by an instinct to the position 

 which they finally occupy. 



As belonging to the same group of phenomena, may we not 

 also signalise the fertilization of the ovum by the spermatozoa, 

 as suggested by Dr. Wyman, and of the plant ovule by the 

 pollen- tube or antherozooid ? Though there is to some extent 

 and in some cases a direct passage prepared for the moving 

 element to use, yet there is almost invariably either direct 

 penetration of the ovum-cell itself or of surrounding tissues. 

 The out-wanderings and in-wanderings of corpuscles through 

 the walls of capillaries and small veins may very well be 

 coupled with and help us to understand the penetrating move- 

 ments of male reproductive corpuscles, carried to such an 

 extent as to affect the whole of the tissues surrounding an 

 ovule or even neighbouring flowers in some plants ;^ and, on 

 the other hand, in animals being so far-reaching as to bring 

 the spermatozoid through the tunics of the ovary and other 

 seeming obstructions as inferred from cases of extra-uterine 

 foetation. 



The ubiquitousness and the power of many parasites of 

 passing through membranes, — e. g. of Gregarin?e, Sarcocystis, 

 Miescheri, Trichina, Bacteria, &c. — is also worth mention in 

 reference to the penetration of tissues and migrations of cells. 



The annelids offer very beautiful subjects for histological 

 study, and in them is to be observed a constant and normal 

 migration of cells, which may have some interest in relation 

 to the migrations of cells above mentioned. The perivisceral 

 cavity of the Oligochset and Polychset annelids is a large 

 space between the muscular body wall and the axial alimentary 

 canal, lined with a more or less complete endothelium, in 

 some Polychaeta ciliated, traversed by thin muscular septa 

 communicating by segmental openings with the exterior, and 

 containing a colourless liquid of oxidizing properties similar 

 to those possessed by solutions of haemoglobin, and in which 



' The fertility of the closed flowers of Viola odoruta, some Campanulce, 

 &c., is perhaps thus explained. 



