178 Transactions of the ViZ^lZ^^im. 



of which averages about 1-2 0th inch, and the transverse about 

 l-25th inch ; the proportion of the former to the latter being thus 

 as 5 to 4. But this proportion is hable to variation; some ova 

 being more elongated (Fig. 1), and others more nearly hemispherical, 

 than ordinary. 



2. The Ova of Fishes, if attached at all, are usually made to adhere, 

 either to each other or to sohd surfaces on which they are deposited, 

 by an albuminous secretion formed around them during their pas- 

 sage outwards from the ovary, just as in the well-known case of the 

 spawn of the Frog. In some instances, however, the connection of 

 the ova with each other is formed by villous appendages, which may 

 either spring from the whole surface of the shell-membrane, as in 

 the Perch, or may be limited to one portion of it, as in the Stichle- 

 hack. Such villous filaments are here found proceeding from the 

 under or flat side of the shell-membrane ; and whilst those arising 

 from the central portion of the area are very short, those developed 

 from its peripheral portion are of considerable length, forming a 

 fringe which extends itself far from the margin of the ovum (Fig. 2); 

 and these tie down the ovum to the subjacent surface by their 

 firm adhesion to it. The Shell-membrane itself appears to me a 

 simple horny pellicle of great transparence, possessing no structure 

 whatever. I have searched iu vain for the fine tubulation detected 

 by Prof. Miiller in the shell-membrane of the egg of the Perch,* and 

 by Prof. Allen Thomson in that of the Salmon and Trout, f And 

 the villous filaments proceeding from it seem mere extensions of the 

 same structureless substance, springing from little papillary eleva- 

 tions of its surface. In Fig. 3 is shown, under a power of 150 

 diameters, a portion of this villous shell-membrane from near the 

 centre of the flat surface ; and in Fig. 4 a marginal portion with its 

 fringe of elongated fibres. I feel confident that these filaments are 

 not tubular, and that they have no epithelial investment ; they 

 cannot, therefore, be in any way likened to the villi of the Mam- 

 malian chorion ; and it is obvious that their function is simply 

 mechanical. 



3. The Micropyle, or aperture in the investment of the ovum 

 through which the spermatozoa penetrate to its interior, was first 

 discovered in the ovum of the Stickleback (Gasterosteus) by Dr. 

 Kansome in 1854, J but independently and almost contempora- 

 neously in the ova of two species of Salmo by Professor Bruch, 

 of Basle.§ Since that date it has been recognized in the many 

 other ova, both Vertebrate and Invertebrate. I doubt, however, if 

 it has ever been so distinctly seen as it can be in the ovum now 



* ' Miiller's Archiv.' for 1854, p. 186. 



t Art. " Ovum," in ' Cyclopc'edia of Anat. and Phys.,' vol. v., p. 100. 

 X See Dr. Kansome, " On the Impregnation of the Ovum of tlie Stickleback," 

 in ' Proceedings of Royal Society,' vol. vii. (1854), pp. 168-172. 

 § 'Zeitschrift fiir Wissensch. Zool.,' Bd. vii., pp. 172-175. 



