420 NATURAL SCIENCE. June. 



plan of structure,, necessitating an exact division of each of its com- 

 ponent parts. It is difficult to imagine such a complicated process 

 as karyokinesis arising suddenly in a small nucleated mass of 

 " Urschleim," or primordial protoplasm. It would seem more natural 

 to imagine the nucleus dividing irregularly at first, which division 

 gradually became more and more precise as the importance of the 

 nucleus as an element in the cell increased. It cannot be considered, 

 however, that amitosis, when it occurs in the Metazoa, is a reversion 

 to a primitive condition. There can be little doubt that it is an 

 acquired process, dependent on some special change in the nucleus 

 or cytoplasm. 



The Relations of Nuclear Division to Reproduction and Segmentation. 



It is well known that the essential process of fertilisation is the 

 conjugation or " fusion " of the head of the spermatozoon, with the 

 nucleus of the ovum,. In order to understand the nature of the 

 phenomenon, it is necessary to study the part played by each of the two 

 elements concerned. The head of the spermatozoon, or sperm- 

 nucleus, is practically a mass of chromatin, with nothing resembling 

 the achromatic network of a resting nucleus. The body, or middle 

 piece, is the centrosome, the nature of which was fully discussed in 

 the first part of this article. The tail consists merely of vibratile 

 protoplasm, and either drops off while the spermatozoon is still in the 

 periphery of the ovum, or may persist for a while in the substance of 

 the egg, eventually becoming detached from the head and finally 

 absorbed. The head itself, after penetrating into the egg, may 

 either [a) be transformed immediately into a resting nucleus with 

 membrane and achromatic network, a condition it maintains through- 

 out, or (b) having first passed into a resting condition, later on its 

 chromatic elements may become collected to form one or more 

 chromosomes, but always a definite number for every species, while 

 the achromatic elements and the membrane simultaneously disappear. 



In Ascaris rnegalocephala, the cldiSsic^X object for research on nuclear 

 structures, the sperm-nucleus always possesses a certain number of 

 large chromosomes in the form of threads or rods. The whole 

 process of fertilisation has been most minutely studied by Van 

 Beneden, Boveri, Carnoy, and others. The egg-nucleus is precisely 

 similar in appearance, and in the number of chromosomes it contains, 

 to the sperm-nucleus. To reach this condition, it has had to undergo 

 important changes and losses, caused by the so-called "reducing 

 division," or extrusion of the polar bodies. The significance of these 

 structures has been interpreted in many different ways, some of which 

 will be briefly touched upon later. It is enough at this point to call 

 attention to the fact that, by a reduction in the amount of chromatin 

 it originally possessed, the egg-nucleus has come to exactly resemble 

 in all outward features the metamorphosed head of the spermatozoon. 

 Passing on now to the actual process of fertilisation, Van Beneden 



