684 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



pelagic eggs the blastodisc 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. Kupffer, 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 

 Meissner first described the aperture in crustaceans and insects (No. 102, p. 272), 

 and Leuckart 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 Ferd. Keber (No. 77) conceives not only this to be the case, but that 

 through it there is an actual outflow of the contents of the egg — 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. 



Meissner, 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 Boeck, 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 Deutoplasm or Food-Yolk. 



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 Ryder, the germ is lateral in A.losa. 



t Bkuch independently discovered the micropyle in the eggs of the trout and salmon (No. 35, p. 172). 



