758 REPOET— 1889. 



former case the mnle pronucleus, in tlie latter Xhe fetnale pronucleus, and the body 

 formed by their fusion is called the segmentation nucleus. These nuclei are so 

 small that it seems almost a contradiction in terms to speak of their magnitude ; 

 rather one might say their miuimitude, for it requires the higher powers of the 

 best microscopes to see them and follow out the process of conjugation. But 

 notwithstanding their extreme minuteness, the pronuclei and the segmentation 

 nucleus are complex both in chemical and molecular structure. From the seg- 

 mentation nucleus produced by the fusion of the pronuclei with each other, and 

 from corresponding changes which occur in the protoplasm of the egg which 

 surrounds it, other cells arise by a process of division, and these in their turn also 

 multiply by division. These cells arrange themselves in course of time into layers 

 which are termed the germinal or embryonic layers. From these layers arise all 

 the tissues and organs of the body, both in its embryonic and adult stages of life. 

 The starting-point of each individual organism — i.e., of each new generation — is 

 therefore the segmentation nucleus. Every cell in the adult body is derived by 

 descent from that nucleus through repeated division. As the segmentation nucleus 

 is formed by the fusion of material derived from both parents, a physical con- 

 tinuity is established between parents and ofispring. But this physicalcontinuity 

 carries with it certain properties which cause the offspring to reproduce, not only 

 the bodily configuration of the parent, but other characters. In the case of Man 

 we find along with the family lilreness in form and features a correspondence in 

 temperament and disposition, in the habits and mode of life, and sometimes in the 

 tendency to particular diseases. This transmission of characters from parent to 

 ofispring is summarised in the well-known expression that ' like begets like/ and 

 it rests upon a physical basis. 



The size of the particles which are derived from the parents, called the male 

 and female pronuclei, the potentiality of which is so utterly out of proportion to 

 their bulk, is almost inconceivably small when compared with the magnitude of 

 the adult body. Further, by the continual process of division of the cells, the 

 substance of the segmentation nucleus is dilfused throughout the body of the new 

 individual produced through its influence, so that each cell contains but an in- 

 finitesimal particle of it. The parental dilution, if I may so say, is so attenuated 

 as to surpass the imagination of even the most credulous believer in the attenua- 

 tion of drugs by dilution. And yet these particles are sufficient to stamp the 

 characters of the parents, of the grandparents, and of still more remote ancestors 

 on the offspring and to preserve them throughout life, notwithstanding the constant 

 changes towhich the cells forming the tissues and organs of the body are subjected 

 in connection with their use and nutrition. So marvellous, indeed, is the whole 

 process, tbat even the exact contributions to recent knowledge on the fusion of the 

 two pronuclei, instead of diminishing our wonder, have intensified the force of the 

 expression ' mafpium hereditatis viysterium.'' 



In considering the question of how new individuals are produced, one must 

 keep in mind that it is not every cell in the body which can act as a centre of 

 reproduction for a new generation, but that certain cells, which we name germ- 

 cells and sperm-cells, are set aside for that purpose. These cells, destined for the 

 production of the next generation, form but a small proportion of the body of the 

 animal in which they are situated. They are, as a ride, marked off" from the rest 

 of the cells of its body at an early period of development. The exact stage at 

 which they become specially differentiated for reproductive pm-poses varies, how- 

 ever, in different organisms. In some organisms, as is said by Balbiani to be the 

 case in Chironomus, they apparently become isolated before the formation of the 

 germinal layers is completed; but, as a rule, their appearance is later, and in the 

 higher organisms not until the development of the body is relatively much more 

 advanced. 



The germ-cells after their isolation take no part in the growth of the organism 

 in which they arise, and their chief association with the other cells of its body is 

 that certain of the latter are of service in their nutrition. The problem, there- 

 fore, for consideration is the mode in which these germ or reproductive cells 

 become influenced, so that after being isolated from the cells which make up the 



