Reproduction and Development. 



431. FORMS OF REPRODUCTION. I. Abiogenesis (Generatio aequivoca, sive spontanea, 

 spontaneous generation). It was formerly assumed that, under certain circumstances, non- 

 living matter derived from the decomposition of organic materials became changed spontaneously 

 into living beings. While Aristotle ascribed this mode of origin to insects, the recent observers 

 who advocate this form of generation restrict its action solely to the lowest organisms. Experi- 

 mental evidence is distinctly against spontaneous generation. If organised matter be heated to 

 a very high temperature in sealed tubes, and be thus deprived of all living organisms or their 

 spores, there is no generation of any organism. Hence, the dictum ' ' Omne vivum ex ovo " 

 (Harvey, or, ex vivo). Some highly organised invertebrate animals (Gordius, Anguillula, Tardi- 

 grada, and Rotatoria) maybe dried, and even heated to 140 C, and yet regain their vital 

 activities on being moistened (Anabiosis). 



II. Division or fission occurs in many protozoa (amoeba, infusoria). The organism, just as is 

 the. case with cells, divides, the nucleus when present taking an active part in the process, so 

 that two nuclei and two masses of protoplasm 

 forming two organisms are produced. The 

 Ophidiasters amongst the echinoderms divide 

 spontaneously, and they are said to throw off 

 an arm which may develop into a complete 

 animal. According to Trembley (1744), the 

 hydra may be divided into pieces, and each 

 piece gives rise to a new individual [although 

 under normal circumstances the hydra gives off 

 buds, and is provided with generative organs], 



[Division of Cells. Although a cell is de- 

 fined as a "nucleated mass of living proto- 



Fig. 623. Fig. 624. 



Fig. 623. Changes in a cell nucleus during karyokinesis. Fig. 624. Typical nucleated cell of 

 the intestinal epithelium, of a flesh-maggot. mc, membrane of cell; mn, membrane of 

 nucleus ; pc, cellular protoplasm, with the radiating reticulum, and the enchylema enclosed 

 in its meshes ; pn, plasma of nucleus ; bn, nuclear filament showing numerous twists. 



plasm," recent researches have shown that, from a histological as well as from a chemical point of 

 view, a cell is really a very complex structure. The apparently homogeneous cell-substance is 

 traversed by a fine plexus of fibrils, with a homogeneous substance in its meshes, while a 

 similar network of fibrils exists within the nucleus itself (fig. 623).] 



[The nucleus of a typical cell is a spherical vesicle, consisting of a membrane containing what 

 is called " achromatin," because it is not readily stained by staining reagents. Flemming has 

 also called it nuclear fluid, or intermediate substance. The achromatin substance is permeated 



