684 PROFESSOR W. C. M‘SINTOSH AND MR E. E. PRINCE ON 
pelagic eggs the blastodise formed after fertilisation is also theoretically constant, but in 
the reverse segment—the animal pole being underneath, and in calm water the germ 
is usually found at this lower pole.* 
As to the function of the micropyle, most authorities are agreed that it is connected 
with the fertilisation of the ovum, affording means, in fact, for the entrance of the 
spermatozoa. Kuprrer, however, calls this generally adopted view into question, and 
doubts whether it has any essential part to play in fecundation (No. 87, p. 179). In the 
ova of lower forms the function named has been universally admitted from the time 
Merssner first described the aperture in crustaceans and insects (No. 102, p. 272), 
and Levckart laboriously worked at the structure and function of this aperture in a 
large variety of insects. The latter, in his elaborate paper, states that he beheld sperms 
not only adhering to the outside of the egg, but entering the micropyle ; and indeed figures 
this phenomenon in the ovum of Melophagus ovinus, a crowd of spermatozoa being 
collected at the external opening, though not more than three or four find entrance. 
In Teleosteans its function appears to be solely that of affording ingress for the fertilising 
element, though Frrp. Keser (No. 77) conceives not only this to be the case, but that 
through it there is an actual outflow of the contents of the ege—the purpose of this 
outflow being to lubricate the canal and favour the entrance of sperms, as well as to 
increase the vacant space within for the reception of the spermatozoa, 
MerssNer, who first described the micropyle in the ovum of the rabbit, thought that 
the aperture only penetrated the vitelline membrane, and that it was effectually closed 
over by the chorion outside (No. 103). A modified view has been put forward by 
Ransom, who was probably the earliest to discern and rightly interpret this aperture in 
osseous fishes.t He was of opinion that a delicate film covered the micropyle, which 
was only ruptured by the entrance of sperms; and more recently Borck, in connection 
with his remarkable theory of osmotic fertilisation, to which we shall refer shortly, 
conjectures that a clear membrane, in the case of Clupea harengus, closes the aperture of 
the micropyle (No. 23, pp. 5, 6). Besides admitting sperms, a small quantity of water 
may also enter, which (water) mingles with certain organic particles, and fills up the space 
between the vitellus and the zona radiata in the extruded ovum. 
THe DrutorLtasm or Foop-Youk. 
Within the egg-capsule is the ovum proper, a spherical translucent mass, largely 
composed of fluid food-yolk. With the food-yolk, which serves for nutrition, there is 
interfused active protoplasm, and this, at an early stage, collects as a delicate film over 
the surface of the yolk-ball; indeed the mature ovum of Teleosteans, before fertilisation, 
exhibits a distinct superficial layer of clear protoplasm, in which minute vesicles and oil- 
* According to RyprEr, the germ is lateral in Alosa. 
+ Brucu independently discovered the micropyle in the eggs of the trout and salmon (No. 35, p. 172). 
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