440 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



pie fission ; but all the varieties are modifications of the simplest form, fission 

 or division. In fission, found only in unicellular organisms and typified in 

 Amoeba, the protoplasm of the single cell, together with the nucleus, becomes 

 divided into two approximately equal portions which separate from one 

 another. In the process no material is lost, and two independent nucleated 

 organisms result, each approximately half the size of the original. The 

 parent has become bodily transformed into the two offspring, which have only 

 to increase in size by the usual processes of assimilation in order themselves 

 to become parents. In higher organisms, even where sexual processes alone 

 prevail in the production of new individuals, the asexual method has per- 

 sisted in the multiplication of the individual cells that constitute the body; 

 embryonic growth is an asexual reproductive process, a continued fission, dif- 

 fering from the amoeboid type in the facts that the resulting cells do not sepa- 

 rate from one another to form independent organisms, but remain closely 

 associated, undergo morphological differentiation and physiological specializa- 

 tion, and together constitute the individual. Likewise in the adult the pro- 

 duction of blood-corpuscles and of epidermis, the regrowth of lost tissues, and 

 the healing of wounds are examples of asexual cell-reproduction. From the 

 standpoint of multicellular growth Spencer and Haeckel have happily termed 

 the process of asexual reproduction in unicellular organisms " discontinuous 

 growth." 



Sexual Reproduction. — Sexual reproduction, or gamogenesis, occurs in 

 unicellular organisms, where it is known as conjugation, and it is the prevail- 

 ing form of reproduction in most of the multicellular forms. In most of the 

 invertebrate and vertebrate animals it is the sole form of reproduction of 

 individuals. In its simple form of conjugation, typified in the minute monad, 

 Heteromita, it consists of a complete fusion of the bodies of two similar indi- 

 viduals, protoplasm and nuclei, followed by a division of the mass into 

 numerous spore-like particles, each of which grows into an adult Heteromita. 

 In the higher infusorian, Paramaeeium, the fusion of the two similar individ- 

 uals is a partial and temporary one, during which a partial exchange of 

 nuclear material takes place • tin- is followed by separation, after which each 

 individual proceeds to live its ordinary life and occasionally to multiply by 

 simple fission. 



In the highly specialized sexual reproduction of higher animals, including 

 man. the individuals of the species are of two kinds or sexes, the male and 

 the female, with profound morphological and physiological differences between 

 them ; in each the protoplasm of the body consists of two kinds of cells, somatic 

 cells and germ-cells, the former subserving the nutritive, muscular, and nervous 

 function- of daily life, the latter subserving reproduction. The germ-cells of 

 the male, called spermatozoa, are relatively small and active, those of the 

 female, called ova, are relatively large and passive; the reproductive process 



consists of a fusi >f a male and a female germ-cell, the essential part being 



a fusion of their nuclei ; and this is followed by continued asexual cell-division 

 and growth into a new individual. Among both plants and animals it is not 



