8 GENERAL INTRODUCTION. 



in this case, as in many others, of two distinct phenomena: first, the introduction into the egg of the paternal hereditary 

 characteristics potentially contained in some unknown manner in the substance of the sperm-nucleus or of the 

 chromosomes into which it resolves itself. Second, the introduction into the egg of a centrosome which gives rise to 

 the mechanism by means of which the egg divides and the hereditary substance is distributed to the resulting cells. 



Van Beneden and Boveri have shown, in the case of Ascaris — and the same is probably true of all other animals 

 and plants — that the substance of the paternal and maternal chromosomes is equally distributed to each of the first two 

 cells (Text-fig. VI.); and there is every reason to believe that this equal distribution of chromatin continues at every 



Fig. VI. 



Fig. VI. — First division of the egg in Arcaris [after Boveri. — Fig. A slightly modified]. 

 A. The chromosomes-halves (cf. Fig. III. D) passing to opposite poles of the spindle. 

 13. Soon after division, the daughter-nuclei reforming. In each daughter-nucleus are four long chromosomes. Two of these on each side are of maternal origin, two of paternal. 



Beside each nucleus lies a granular archoplasni-mass (A) derived from the corresponding aster. The centrosome (C) in each has already divided into two preparatory to the 



next division of the cell. PB, the polar bodies. 



succeeding cell-division. In the adult body, therefore, every nucleus of all the myriad cells of which it is composed 

 contains chromatin derived from both the parents. Thus we are enabled to understand, from a physical point of view, 

 that marvellous interweaving of paternal and maternal traits that characterizes the constitution of the offspring. 



Maturation. Formation of the Polar Bodies. — Before the final union of the germ-cells each of them undergoes a 

 series of changes, collectively known as maturation, by which it is prepared for the union with its fellow. These changes 

 affect especially the structure of the nucleus, and in the case of the egg-cell involve the formation of the polar bodies, — 



P B 1 



Fig. VII. — Formation of the polar bodies in Toxopneustes. From life. 365 diameters. 

 .\. Preparatory charge of form in the germinal vesicle. 



B. The same egg two hours later. The germinal vesicle has disappeared, the first polar body has been extruded, and the second is just forming. The polar amphiaster is clearly 



visilile, but the chromosomes (dyad groups) are invisible in life. 



C. The same egg one hour later. Both polar bodies are still attached to the periphery of the egg. The chromosomes remaining in the egg (i.e., a remnant of the chromatin of the 



germinal vesicle) have given rise to the egg-nucleus (K). 



