TKANSACTIONS OF SECTION U. 761 



bodies, I summarised my conclusion as follows: 'Hence, in llie development of 

 each individual, a morpliolo^it-al specialisation occurs both in internal structure and 

 external form by which distinctive characters are conferred, so that each man's 

 structural individuality is an expression of the sum of the individual variations of 

 all the constituent parts of his frame.' 



As in that paper 1 was discussing the subject only in its morphological 

 relations, I limited myself to that aspect of the question ; but I might with equal 

 propriety have also extended my conclusion to other aspects of man's nature. 



Intimately associated, therefore, with the conception of Heredity— that is, the 

 transmission of characters common to both parent and oll'spring — is that of Varia- 

 bility — that is, the appearance in an organism of certain characters which are unlike 

 those possessed by its parents. Ileiedity, therefore, may be defined as the perpe- 

 tuation of the like ; "N'ariability, as the production of the unlike. 



And now we may ask, Is it possible to offer any feasible explanation of the 

 mode iu whicli variations in organic structure take their rise in the course of 

 development of an individual organism? Anything that one may say on this head 

 is of coarse a matter ot speculation, but certain facts may be adduced as offering 

 a basis for the construction of an hypothesis, and on this matter Professor Weis- 

 mann makes a number of ingenious suggestions. 



Prior to the conjugation of the male and female pronuclei to form the segmen- 

 tation nucleus a portion of the germ-plasm is extruded from the egg to form what 

 are called the polar bodies. Various theories have been advanced to account for 

 the significance of this curious phenomenon. Weismann explains it on the hypo- 

 thesis that a reduction of the number of ancestral germ-plasms in the nucleus of the 

 egg is a necessary preparation for fertilisation and for the development of the 

 young animal. lie supposes that by the expulsion of the polar bodies one half the 

 number of ancestral germ-plasms is removed, and that the original bulk is restored 

 by the addition of the male pronucleus to that which remains. As precisely cor- 

 responding molecules of this plasm need not be expelled from each ovum, .similar 

 ancestral plasms are not retained in each case ; so that diversities would arise even 

 in the same generation and between the offspring of the same parents. 



Minute though the segmentation nucleus is, yet microscopic research has shown 

 that it is not a homogeneous structureless body, but is built up of different parts. 

 Most noteworthy is the presence of extremely delicate threads or fibrils, called the 

 chromatin Jilaments, which are either coiled on each other, or intersect to form a 

 network-like arrangement. In the meshes of this network a viscous — and, so far 

 as we yet know, structureless — substance is situated. Before the process of division 

 begins in the segmentation nucleus these filaments swell up and then proceed to 

 arrange themselves at first hito one and then into two star-like figures before the 

 actual division of tiie nucleus takes place.' It is obvious, therefore, that the mole- 

 cules which enter into the formation of the segmentation nucleus can move within 

 its substance, and can undergo a readjustment in size and form and position. But 

 this readjustment of material is, without doubt, not limited to those relatively coarse 

 particles which can be seen and examined under the microscope, but applies to the 

 entire molecular structure of the segmentation nucleus. Now it must be remembered 

 that the ceils of the embryo from which all the tissues and organs of the adult body 

 are derived are themselves descendants of the segmentation nucleus, and they will 

 doubtless inlierit from it both the power of transmitting definite characters and a 

 certain capacity for readjustment both of their constituent materials and the 

 relative positions which tliey may assume towards each other. One might conceive, 

 therefore, that if in a succession of organisms derived from common ancestors the 

 molecular jiarticles were to be of the same composition and to arrange themselves 

 in the segmentation nucleus and in the cells derived from it on the i-ame lines, 

 these successive generations would be alike ; but if the lines of adjustment and the 

 molecular constitution were to vary in the different generations, tnen the products 



' The observations more especially of Flemniing, K. Van Rencden, Strasburger, aud 

 Carnoy may be referred to in connection with the changes which take place in nuclei 

 prior to and in connection with their division. 



