284 THE CELL 



by means of which they move about independently in the water. 

 They are of especial interest, for they show us how, by means of 

 gradual differentiation and division of labour in opposite directions, 

 they have developed more highly differentiated forms, namely, 

 typical eggs and typical antherozoids. 



Swarm-spores are small, motile, naked cells, generally pear- 

 shaped (Figs. 154, 155, 157, 158). The pointed end is anterior 

 and goes in front, whilst the spore moves through the water ; it 

 consists of hyaline protoplasm, and frequently contains a red or 

 brown pigment spot (the eye-spot) ; the remainder of the body is 

 hyaline, or coloured green, red, or brown with 

 colouring matter, according to the species ; it con- 

 tains one or two contractile vacuoles (Fig. 154). 

 The swarm-spore moves along by means of flagella, 

 which spring from the hyaline anterior portion ; 

 there are generally two flagella (Fig. 154), but 

 s varm-spore of sometimes there is only one; occasionally there are 

 o- four or more (Fig. 14). 



r The swarm-spores are derived at certain times 

 from the contents of a mother-cell, either by means 

 of repeated bipartitions, or by the splitting up of the mother-cell 

 into several portions (pp. 232-234). When division into two 

 occurs, the number of swarm-spores is small, being 2, 4, 8, or 

 16 ; when, however, many cells are produced, the number is very 

 great, for in that case the mother-cell is of considerable size, and 

 may produce as many as from 7,000 to 20,000 daughter-cells. 

 When the wall of the mother-cell ruptures at one place, the broad 

 end of the swarm-spore escapes first to the exterior. 



There are two kinds of swarm-spores, which are developed at 

 different times. The one kind multiply asexually, giving rise to 

 young Alga, whilst the others require fertilisation. The mother- 

 cell, from which the former are derived, is termed by botanists the 

 sporangium, that giving rise to the latter gametangium. 



We will only consider sexual spores or gametes here. In many 

 : the lower Algae conjugating swarm-spores (Fig. 155 a, b, c, d) 

 cannot be distinguished from one another in any respect, either as 

 regards their sizes, mode of movement, or behaviour (Ulothrix, 

 Jtryopeu, Botrydium, Acetabularia, etc.). On the other hand, in 

 ar species sexual differences develop, which enable us to dis- 

 tinguish between male and female gametes. In the first case wo 

 speak of isogamous, and in the second oogamous fertilisation 



