484 



BIOLOGY AND HUMAN LIFE 



ff — 7 V/ J\J K 



Fig. 202. Conjugation in Paramecium 



There are two nuclei: a, the small one {micro- 

 nucleus): and b, the large one {macronuclcus). 

 A: Two individuals become attached and their 

 micronuclei begin to divide. B : The half nuclei 

 divide a second time. Of the four units result- 

 ing, three are called polar bodies, c, and the 

 fourth is a germ nucleus, g, which again divides. 

 C: The germ nuclei are interchanged, one of 

 each pair passing over to the opposite animal. 

 D: Completion of interchange. E: Same, further 

 enlarged. F: The active germ nucleus fuses with 

 the stationary one. G: Same, enlarged. The ma- 

 cronuclcus has broken up and disappeared. After 

 the restoration of the micronucleus through 

 fusion, F, the two individuals float apart. H: 

 The new micronucleus, d, breaks up into two. /: 

 Each portion splits again. /: A third division. 

 K: Four of the nuclei become new macronuclei 

 and four remain micronuclei. The rest of the 

 protoplasm divides and four individuals result, 

 each with micronucleus and macronucleus. ( From 

 Calkins, after Hertwig, and Maupas) 



the sperm cells and the 

 female the egg cells. 

 Among certain inverte- 

 brates, however, the fer- 

 tilized eggs are cared for 

 in some way. Among 

 crayfish and lobsters, for 

 example, the eggs have 

 a sticky surface and at- 

 tach themselves to the 

 abdominal legs of the 

 mother and remain at- 

 tached until they have 

 hatched. Among clams 

 the eggs remain inside 

 the mantle cavity of the 

 mother and so receive 

 considerable protection 

 until the embryos are 

 able to swim away. 



356. Reproduction in 

 fishes. The female fish 

 deposits the eggs in quiet 

 places at the bottom of 

 the sea, near shore, or in 

 quiet pools of rivers. 

 Then the male fish swims 

 over the eggs, dropping 

 out a quantity of milt, 

 or semen, as the fluid 

 containing sperm cells is 

 called. These cells swim 

 about in the water, and 

 fertilization takes place 

 very much as in the rock- 

 weed (Fig. 194). 



