GROWTH, REGENERATION AND REPRODUCTION 1117 



terminating the existence of the species by a process of regeneration 

 or reproduction in mass. A special group of organs is set aside for 

 the formation of what might be termed in brief the germ-plasm, a 

 specialized substance which is capable not only of reforming a par- 

 ticular type of cell but also of reproducing the counterparts of all 

 cells within a single entity which then takes the place of the one gone 

 out of existence. The organs to which this function is assigned are the 

 reproductive organs. Their chief product is the ovum, a cellular 

 unit containing the germ-plasm. In this germinal cell begins the 

 development of every new living entity. 



In the majority of living forms, however, the ovum is not capable 

 of undergoing division unless it is energized by another cell which is 

 known as the sperm-cell or spermatozoon. Thus, reproduction may 

 be either asexual or sexual. The former process or parthenogenesis 

 is confined to some of the lower and simpler types of life, while the 

 latter is peculiar of all higher forms. In sexual reproduction the ovum 

 represents the female element, and the spermatozoon the male element. 

 The former consists essentially of cytoplasm which contains a consider- 

 able quantity of nutritive material, while the latter is principally 

 composed of nuclear substance. The essence of this mechanism is 

 the meeting and fusion of these two elements into a single one from 

 which a new individual is then developed. This fusion by means 

 of which two independent units are blended into one, constitutes 

 the process of fertilizations fecundation. 



In explanation of this interaction two theories have been promulgated, namely, 

 one emphasizing the importance of the spermatozoon and one emphasizing that 

 of the ovum. The advocates of the former are known as animalculists and hold 

 that the spermatozoon is a complete animal en minuture, possessing all the char- 

 acteristics of the parent but lacking a fertile medium in which to grow. This 

 medium it seeks and finally attains by virtue of its inherent power of movement. 

 The advocates of the second view, who are known as ovists, believe that the ovum 

 contains all the essentials of the full grown organism, but needs a stimulus to make 

 it develop. This impetus is given to it by the spermatozoon. In accordance with 

 this view the ovum may be likened to the bud of a plant which unfolds its leaflets and 

 begins to grow as soon as the proper stimuli have been received by it. Both these 

 conceptions are based upon the idea that either the spermatozoon or the ovum are 

 preformed and hence, they may be collectively referred to as the theory of preforma- 

 tion. Subsequent investigation, however, has shown that the spermatozoon as 

 well as the ovum are but single cells and have a perfectly definite life history. 

 Both originate in the germinal cells of two separate individuals, and both pass 

 through definite preliminary changes before they actually attain their maturity. 

 Furthermore, while the part played by them in fertilization is not exactly the same, 

 their purpose is identical, i.e., both strive to produce a new individual. Conse- 

 quently, neither can be said to be more important than the other. 



It must be admitted, however, that we are still in ignorance regarding the physi- 

 ological principle underlying this fusion of the germinal elements. Harvey and 

 others have advocated the view that the ovum is animated by the spermatozoon 

 and is thereby made to develop. This idea is embodied in the so-called dynamic 

 theories of Spencer, Biitschli, and Hertwig, which assume that protoplasm 

 becomes increasingly inactive and finally requires fertilization to imbibe it with 

 a new force developed under different conditions. This process, therefore, could 



