REPRODUCTION AND SEX 



451 



the intimate union of the reproductive units (gametes), to which 

 Weismann applied the useful term Amphimixis. But the process 

 of reproduction has also to take note of early development ; and, 

 also, of such antenatal relation as may be between parent and off- 

 spring. Nor can the study of reproduction be separated off from that 

 of heredity — that relation of organic continuity between successive 

 generations, which secures the begetting of like by like, and yet 

 allows of the emergence of those novelties or variations which 

 become so significant in further evolution. 



MODES OF REPRODUCTION. — In one-celled organisms, 

 whether Protists, Protophytes, or Protozoa, the unit divides by 



Fig. 55. 



The Female Reproductive Organ of the Adder's -tongue Fern (Ophioglossum). 

 After Bruchmann. At the base there is an egg-cell or ovum (OV), and in 

 the entrant canal a secretion of mucilage (M). Some ciliated sperm-cells 

 (SP) are attracted to the entrance, and one of them will normally unite 

 with the ovum and effect fertilisation. 



fission, budding, or spore-formation into two or more parts, and 

 each of these parts grows into the likeness of the whole. Yet even 

 at this low level of unicellular life it often happens that two indi- 

 viduals combine to form one. The single product of this total 

 conjugation may rest for a time, but sooner or later proceeds to 

 divide into many. In many higher Protozoa, there arise specialised 

 reproductive units (gametes) — sometimes even dimorphic — and 

 these unite by pairs, forming a "zygote", which either grows into 

 the original form or divides into many individuals. Very suggestive 

 also is the partial conjugation of such ciliated Infusorians as the 

 Slipper Animalcule (Paramoecium), where there is an exchange of 

 nuclear elements and then a separation of the conjugating pair. 



