THE TISSUES. 35 



egg protoplasm. The sperm cell centrosome undergoes division, 

 so that a central spindle is formed (Figs. 14-16). 



At this stage of the fertilization the process of maturation 

 is usually completed, and the egg nucleus has become vesicular 

 (Fig. 16). The sperm nucleus now begins to swell and become 

 also vesicular, and approaches the egg nucleus. The sperm 

 centrosome and central spindle at the same time become closely 

 related to the sperm nucleus (Figs. 15, 16). Both nuclei 

 become larger, and as they approach one another the radiation 

 of the egg centrosome becomes less conspicuous. The sperm 

 radiation becomes more and more prominent, spreading over 

 the whole cell. Finally the egg radiation vanishes, since the 

 functions of the protoplasmic striations, as well as of the egg 

 centrosome, are after the extrusion of the two polar bodies 

 ended (Figs. 16, 17). 



The radiation arising from the spermatozoon enters into 

 combination with the nuclear framework and the last chromo- 

 somes of the egg nucleus. At this moment the process of fer- 

 tilization as such is completed. Both nuclei undergo the first 

 stages of indirect division and give rise to a mother star (Figs. 

 18, 19). 



The further process is not different from the ordinary mi- 

 totic division. This karyokinetic figure should form nuclei, of 

 which each contains an equal number of male and female 

 nuclear segments. The number of chromosomes in the fer- 

 tilized egg equals the sum of the chromosomes of the ripe egg 

 and those of the spermatozoon that is, the original full number 

 of chromosomes which is characteristic of the somatic cells of 

 the animal. 



B. THE TISSUES. 



The lowest animal organisms (protozoa) are unicellular 

 structures. Since there is only one cell, this must carry out all 

 the life functions. More highly developed animals are made 

 up of many cells (metazoa), which all arise by a division of one 

 single cell i. e., the fertilized egg. These cells are quite similar 

 in their early embryonic state ; and there is an almost spherical, 



