ORIGIN AND DEVELOPMENT OF THE LIVING BEING 7 



Nothing is known concerning the origin of the first living 

 being on earth, from which all others are descendants. 



The morphological phenomena of growth and develop- 

 ment of a living being are the increase in cells and the 

 development of form. 



Cells multiply by fission. 



In this process, the nucleus is divided into two nuclei ; after 

 this the protoplasm divides into two parts, each part surrounding 

 one of the daughter-nuclei. The nuclear division is either direct 

 by die fission of the nucleus or indirect. The indirect division 

 takes place as follows: First the nuclear framework changes to a 

 thick skein-like fibre; this fibre divides, by transverse section, into 

 a number of segments, and each segment splits longitudinally into 

 two halves, each of which goes to build up one of the daughter- 

 nuclei. In this indirect nuclear division, the centrosomes have a 

 definite influence. These structures lying in the protoplasm near 

 the nucleus, in the form of two or more granules, divide, previous 

 to the nuclear division, into two parts and these, by means of 

 fibrilles which proceed from them, direct the course of the separat- 

 ing nuclear segments. (For details see text-book of Histology.) 



The formation of a new organism results in one of two 

 ways. Either some cells separated from the mother organ- 

 ism by cell division continue to exist by themselves inde- 

 pendently and grow (asexual reproduction, reproduction 

 by fission or budding); or two cells, derived from one or 

 two sexually different living beings of the same species, unite 

 to form one living being (sexual reproduction, union of egg- 

 and sperm-cell). 



The union of egg-cell and sperm-cell is the fertilization. 

 In this the nuclei of the cells unite into one nucleus. By 

 means of cell division, accompanied by cell differentiation, 

 the new organism grows from the fertilized egg. 



In many species of living beings (e.g. vascular cryptogams, 

 hydromedusa) there is an alternation of generation in which one 

 generation propagates itself asexual ly, and the other sexually. 



The morphological development of the individual organism 

 (ontogeny) results in the development of the daughter- 

 organism, derived from a single cell, or from the union of 

 egg-cell and sperm-cell, into a form similar or nearly similar 



