24 ZOOLOGY FOR MEDICAL STUDENTS chap. 



individual known as the zygote. In this particular instance the biflagellate 

 cells are gametes, the young microsphere is a zygote. 



It is customary to use the expression sexual " reproduction " but it 

 should be realized from the beginning that the sexual act— syngamy — is 

 not in itself reproduction : it is in fact the opposite for it involves not an in- 

 crease but a decrease in the number of cell-individuals. At the same time 

 it is usual to find the sexual act intimately associated with an increase 

 in the number of individuals. In the case of Polystomella as in many 

 others this increase takes place immediately before the act of syngamy 

 ^n the production of the very numerous gametes. 



Looking back upon the life-history of Polystomella we see in it a good 

 example of what is called alternation of generations— generations of 

 individuals which reproduce sexually (megalospheric) alternating ' with 

 others (microspheric) in which the reproduction is asexual i.e. unac- 

 companied by syngamy. We also see why it is that the sexual individuals 

 are much more numerous (about 30 to i) than the asexual — because in 

 their case enormous wastage takes place owing to the act of syngamy 

 depending on the small chance of (i) two individuals in the same neigh- 

 bourhood producing gametes at the same time and (2) the gamete from 

 the one happening to come into immediate proximity with a gamete 

 from the other. , 



The Foraminifera in general are characterized especially by (i) the 

 slender thread-like pseudopodia with their tendency to fuse together into a 

 network, (2) the presence of a shell or skeleton which may be composed 

 of horny, or siliceous (= flinty) or, as is much more usual, calcareous 

 secreted material, or may on the other hand be built up of foreign 

 particles such as grains of sand or fragments of the skeletons of other 

 minute animals, and (3) by the differentiation of the individuals into two 

 types — asexual microspheric and sexual megalospheric. 



They are typically marine. Many of them creep about on the bottom 

 or on seaweeds, like Polystomella ; others are pelagic (i.e. inhabiting the 

 open ocean), forming an important part of what is known as the plankton 

 i.e. the drifting population of organisms too small or too feeble to travel 

 about by the activity of their own movements. In the pelagic Foramini- 

 fera the cytoplasm commonly contains large numbers of vacuoles filled 

 with a watery jelly of slightly less specific gravity than the sea-water, 

 which serves to keep the specific gravity of the creature approximately 

 the same as that of the surrounding water in spite of its heavy skeleton. 

 These pelagic Foraminifera play an important part in the formation of 

 submarine deposits. They exist in countless myriads in the upper layers 



