DEVELOPMENT OF CELLS. 171 



that is, the adding as of additional forces, — we have in the seed 

 either a noticeable decay or a complete loss of vitality, or 

 such an insufficiency of vitality that the growth can be but 

 feeble and partial, even if suitable conditions for its growth 

 and development are now presented it. Among our domestic 

 animals we have additional illustrations in the effects on fecun- 

 dity following insufficient feeding, full feeding and, perhaps, 

 over- feeding. Among the Arabs a year of scarcity is said to 

 be followed by one of almost complete barrenness among their 

 flocks ; and all breeders of sheep are aware of the importance 

 of abundance of food in influencing the production of twins. 



Generation is the actual contact of two peculiar cells and 

 the union of their forces. On the sufficiency or insufficiency 

 of these forces, and on the relative quality of the concrete 

 force presented by the one parent or the other, depends very 

 much the future progeny. The changes following the union 

 of these two generative cells naturally fall under the head of 

 development, for with the conjunction of the two forces the 

 act of generation is accomplished. 



DEVELOPMENT. 



In the sperm we have a development taking place in the 

 mother-cell and in the nuclei, but this ceases with the forma- 

 tion of the spermatozoon. It is the function of this cell to 

 add force, and influence other creations. In the germ, on the 

 contrary, we have a development going on to an almost 

 unlimited degree. The mammalian ovum is, at the time it is 

 fitted for fecundation, far beyond the simple cell in complex- 

 ity. It contains various contents, granular in form, and dis- 

 tinct membranes enveloping these and the primary cell and 

 its nucleus, — the vesicle and the germinal spot. The ova of 

 various insects, as remarked with reference to the silk-moth, 

 can develop into intermediate embryonic forms quite gen- 

 erally, while a very small fraction can continue to the extent 

 of the completed caterpillar. The function of the ovum is, 

 therefore, to receive forces up to the extent sufficient for its 

 development iuto the state in which it can maintain of itself 

 its equilibrium with its surroundings. 



Parthenogenesis, or the alternation of generation, seems a 

 striking instance of this law of development. The whole 



