MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 41 



soon observed, either completely or at least partially, in numerous other 

 animals also in Coelenterates and Worms (NUSSBAUM, VAN BENEDEN, 

 CARNOY, ZACHARIAS, BOVERI, PLAINER), and in Molluscs and Verte- 

 brates. As regards the last, it has been possible to follow accurately 

 in the case of Petromyzon the penetration of a single spermatozoon 

 into the egg through a special preformed micropyle in the vitelline 

 membrane (CALBERLA, KUPFFER, BENECKE, and BOHM). Likewise in 

 the Amphibia, proof has been brought forward that after fertilisation 

 a sperm-nucleus is formed at the animal pole, and that, surrounded by 

 a pigmented area, derived from the cortex of the yolk, it moves to- 

 ward another more deeply imbedded nucleus (egg-nucleus), and fuses 

 with it (O. HERTWIG, BAMBEKE, BORN). In Mammals the fertilisa- 

 tion takes place in the beginning of the oviduct. Evidence has also 

 been produced in their case that after the liberation of the polar 

 cells two nuclei are temporarily to be seen in the egg-cells, and that 

 these unite in the centre of the egg to form the cleavage-nucleus 

 (VAN BENEDEN, TAFANI). 



This is the proper place in which to mention briefly the so-called 

 micropyle. In many animals (Arthropods, Fishes, etc.) the eggs are 

 enclosed before they are fertilised in a thick firm envelope, which 

 is impenetrable for spermatozoa. Now, in order to make fertilisation 

 possible, there are found in these cases at a definite place on the egg- 

 membrane sometimes one, sometimes several, small openings (micro- 

 pyles), at which the spermatozoa accumulate in order to glide into 

 the interior of the egg. 



The egg of Nematodes has for several years rightly played an 

 important role in the literature of the process of fertilisation. But 

 this is especially true for the egg of the Maw-worm of the Horse 

 (Ascaris megalocephala), which VAN BENEDEN has made the subject 

 of a celebrated monograph. It is an excellent object, in so far 

 as it not only can be had for study everywhere and at all seasons of 

 the year, but also allows one to follow step by step, in the most 

 accurate manner, the penetration and subsequent fate of the sper- 

 matozoon. Since, moreover, the process of fertilisation in Ascaris 

 megalocephala presents many peculiarities in its details, an extended 

 presentation of them is both warranted and desirable. 



In the case of this Worm, in which the sexes are separate individuals, 

 there is a copulation, and the fertilisation of the egg takes place within 

 the sexual passages of the female. In one region, which is expanded 

 into a kind of uterus, mature spermatic bodies are met with in great 

 numbers. The appearance of these differs greatly from that which 



