[100] OVUM. 



g enus was not entirely homogeneous, but was 

 marked through its thickness with fine lines set 

 perpendicularly to the surface. Vogt observed 

 a similar structure, and described it more fully 

 in the Salmonidae.* More recently attention 

 has been particularly called to it by the fuller 

 description of the structure of the egg cover- 

 ings in the Perca fluviatilis by Professor 

 Miiller of Berlin. In this fish Miiller described 

 the radiated lines as produced by fine tubes 

 which pierce the whole thickness of the ex- 

 ternal membrane, beginning with cup or funnel- 

 shaped dilatations on the exterior, preserving a 

 nearly equal diameter throughout, and ter- 

 minating on the inner surface.f The tubes 

 have a slight spiral winding as they pass 

 through. That they are really hollow tubes 

 Miiller ascertained by finding that he was able 

 to press portions of the coloured oily contents 

 of the yolk through them. Miiller farther ob- 

 served, that in the perch each tube is set in a 

 small prism, which terminates by a hexagonal 

 end on the outer surface. According to Dr. 



Fig. 67 *. 



Part of the ovarian ovum of the Salmon. 



Semi-diagrammatic view of the section of a por- 

 tion of the yolk, porous membrane and external 

 layer of cells iu an ovarian ovum of the salmon of 

 3o" in diameter, a, portion of the yolk substance 

 showing the various granules, granular and nu- 

 cleated corpuscles, and oil globules composing it ; 

 h, section of the porous or dotted external mem- 

 brane ; c, portion of the outer surface of the same 

 turned towards the observer so as to show the 

 punctated or dotted marking produced by the ex- 

 ternal apertures of the fine canals which run through 

 the membrane ; d, the flat surfaces of the nucleated 

 cells (epithelial or granular) which line the ovi- 

 capsule, between which and h they are seen edge- 

 ways lying close along the outer surface of the 

 dotted membrane ; a granular or dotted appearance 

 in the contents of these cells seems to indicate their 

 conversion into the dotted membrane, which is pro- 

 bably formed in successive layers from the exterior. 

 The diameter of these cells is -j^", that of their 

 nuclei 33 L;. 



* Embryoge'nie des Saumons, Neufchatel, 1842. 

 t Miiller's Archiv. for 1854, p. 186. 



Ransom's observations, however, it appears 

 that the structure described by Miiller in the 

 perch is peculiar to that fish, and belongs 

 only to an outer covering superadded to the 

 surface of the dotted membrane, which last 

 resembles in all respects that of other fishes. 

 This outer covering appears to be of cellular 

 origin ; and Dr. Ransom thinks it may be due 

 to the separation of the tunica granulosa 

 along with the ovum. The diameter of these 

 tubes in the perch is about 10 \ uo ". In 

 most other fishes the fine lines which ap- 

 pear to be tubular are much smaller. I have 

 observed them in several fishes, and have 

 rarely found more than ten of these tubes 

 in the breadth of T - 1 7nT // , and the tubes them- 

 selves or double lines bounding them were not 

 more than ^&o<j" or ^oW in breadth. In 

 looking at the flat surface of the membrane the 

 ends of these tubes give the appearance of a 

 finely dotted structure to the membrane. It is 

 quite possible, however, even where they are 

 finest, to perceive the circle or lumen of the tube 

 by using a high magnifying power ; and I have 

 thought that I could also in the salmon perceive 

 a hexagonal marking of the intervals between 

 thepores(see#g.68*D); but in this fish the size 

 of the pores is only a third of that of the tubes 

 in the perch as described by Miiller, and the 

 structure must be of a different kind accord- 

 ing to Ransom's observation. All recent ob- 

 servers have recognised this structure in the 

 external membrane of the fish's ovum. Miiller 

 conceived that the tubes he had observed in 

 the perch might be connected with the intro- 

 duction of the spermatozoa into the ovum ; 

 but Dr. Ransom does not find these tubes to 

 pass entirely through the outer membrane of 

 the perch's ovum, and has observed that the 

 part of the true vitelline or dotted membrane 

 which admits the spermatozoa is destitute of 

 the additional layer ; and it will immediately 

 be shown that in all fishes a special and more 

 direct passage for the admission of these bodies 

 through the dense membrane is provided, con- 

 stituting the second peculiarity of structure 

 in the covering of the Fish's ovum before re- 

 ferred to. 



The interesting discovery of an aperture in 

 the external membrane of the ovum of osseous 

 fishes is due to Dr. Ransom of Nottingham, 

 who observed it first in two species of Stickle- 

 back or Gasterosteus, and afterwards in other 

 fishes. This author made the farther interest- 

 ing observation in the first-mentioned fish, 

 that in impregnation the spermatozoa entered 

 the ovum only through this aperture or mi- 

 cropyle. As this is the first instance in 

 which the existence of this aperture and its 

 relation to the process of fecundation have 

 been ascertained by direct observation in a 

 vertebrate animal, I will describe it more fully 

 from Dr. Ransom's paper to the Royal So- 

 ciety of London*, and from farther inform- 

 ation which he has obligingly furnished to me 

 in private. I may also mention that I have 

 fully confirmed Ransom's observations as to 



* Proceedings of Roy. Soc. 1854, Nov. 23rd. 



