OVUM. 



[109] 



the nature of this structure in the mature 

 ovum of Anodonta. 



In this family of Mollusca the micropyle 

 forms a small but very apparent funnel-shaped 

 projection from the surface of the outer mem- 

 brane ; and its hollow nature may easily be 

 ascertained by the fact that the fluid and 

 granular yolk-substance may be forced through 

 it from within. The yolk ball is placed ex- 

 centrically within the vitelline membrane, the 

 inner surface of which it touches just at the 

 place where the micropyle is situated. 



From a variety of observations, it has been 



Fig. 73*. 



Structure and Fonnation of Ova in Acephala. 

 (From Lecaze Duthiers.} 



a. Portion of the ovary with three pediculated 

 ovicapsules and contained ova from Cardium rusti- 

 cum, magnified 400 diameters ; the micropyle is 

 afterwards formed at the place where the pedicles 

 are detached from the secreting coeca of the ovary. 



b. Unripe ovum of Spondylus gaederopus magni- 

 fied 170 diameters, showing the remains of the cap- 

 sule at the upper part, and the projection of the 

 vitelline membrane at the same place where the 

 micropyle is situated. 



c. Ripe ovum of the same burst by pressure, 

 showing the escape of some yolk granules through 

 the micropyle and into the space between the yolk 

 and the outer membrane. In^this and the previous 

 figure the double state of the macula is represented. 



shown that the micropyle of the Acephalous 

 Mollusca owes its origin to the early pediculated 

 attachment of the ovum. This has been fully 

 brought out by the observations of Hessling 

 in Unio and Anodonta, of Leydig in Venus, 

 and of Lecaze Duthiers in Cardium and some 

 other genera.* From these observations it 

 appears that the ova first arise in the ova- 

 rian stroma by the formation of the ger- 

 minal vesicles, as in most other animals, each 

 vesicle possessing a distinct single macula. 

 These vesicles come very soon to be surrounded 

 by some of the primitive or finely granu- 

 lar yolk, which gradually increases in quantity. 

 These parts are from a very early period en- 

 closed by a membrane which may be regarded 

 as vitelline, but which is differently disposed 

 from that in any of the animals previously re- 

 ferred to ; for, instead of having a regular and 

 complete spheroidal or vesicular form, this 

 membrane is elongated at one part into a 

 pedicle, so as to give the whole of the early 

 ova a pyriform shape, and so as to attach them 

 to the ovarian substance by the pediculated 

 parts of the vitelline membrane. In Venus de- 



Fig. 74*. 



Ovarian ova of Venus decussata. (From Leydig.') 



a. A group of five ova in their earliest stage 

 projecting from the ovary in their pediculated 

 capsules : the germinal vesicles with single macula, 

 the vitelline granules, vitelline membrane, and 

 ovicapsule are all distinct. 



b. Two ovicapsules within which at a more ad- 

 vanced stage the ova have become detached from 

 their pedicles, the remains of which at the upper 

 ends of the ova form the micropyle. A considerable 

 amount of albumen has been deposited between the 

 ovum and the ovicapsule. 



* Ann*! des Scien. Nat 1854, ii. p. 155. 



