372 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



from the observations of Roux that the sperm cell is iio-w to be regarded 

 as more than a mere nucleus, that it contains l)oth uuclein and para- 

 iinclein. 



Intercellular forces. — The forces within the different ])ortions of the 

 cell lead us to consider those which must exist between different cells. 

 This is an obscure question at present; but, as I have observed in the 

 close of the second lecture, it is an extremely important one in connec- 

 tion with the problem of heredity. 



As Prof. Wilson writes: "My own conviction steadily gro\rs that the 

 cell is not a self-regulating mechanism in itself, that no cell is isolated, 

 and that AVeisman's fundamental j)ropositi()n is false." 



It is a long step between an a priori conviction and the demonstra- 

 tion by experiment of a correlation of force-^ between the cells. This 

 seems to me a most important lield of experiment. We have seen in 

 Maupas's work that the cojitact of two infusoria initiates a rajiid 

 vSeries of internal changes; we have only to c(mceive of analogous 

 changes taking place when two cells are not in actual contact, as in 

 the phenomena of previous fertilization leferred to in my second lec- 

 ture. Hertwig and others have shown how gravitation is related to 

 cell activity. Roux has destroyed half an embryo with a hot needle in 

 the first stages of segmentation and followed the other half through 

 the stages of subsecpient development. Another clever experimenter 

 has turned fertilized ova u})side down during the early stages of 

 develo])ment, and shown how the protoplasmic pole and yolk pole 

 forcibly change places. Driesch has traced the connecti(m and mean- 

 ing of the first plane of cleavage in the embryos of echinoderms, and 

 has succeeded in raising a small adult from half an embryo artificially 

 se])arated during the first cleavage stage. Wilson, in the larva of 

 Nereis, has shown liow a <'ertain stage of division in one group of cells 

 affects all the other groups. All these experiments are in the line of 

 determining the relations which exist between internal cell forces and 

 other natural forces. What we must now seek to deternune is the 

 relation of cell to cell throughout the body, in connection with the 

 phenomena of heredity. 



Convlutiious. — Perhaps the most impressive result of our review of 

 recent researches in evolution and heredity is the uniformity of life 

 processes throughout the whole scale of life from the infusoria to man. 

 The most striking analogy is that seen in the laws of fertilization and 

 conjugation, which are shown by Maupas's researches to have been 

 established substantially in their present form at a very early period in 

 the evolution of living organisms. Such uniformity furnishes a powerful 

 argument for the advocates of the study of biology as an introduction to 

 the applied science of medicine. Much that is now entirely omitted 

 from medical education, because it is considered too remote, is in reality 

 at the very roots of the science. To understand the disorders of life 



