PHYLUM PROTOZOA 



6l 



macronucleus (Fig. 33, mi.), grows larger, then lengthens, 

 forming a spindle (Fig. 40, a), and subsequently divides into 

 two (b). These immediately divide again without the inter- 

 vention of a resting stage. The resultant four nuclei (c) have 

 been compared to the four sper- 

 matozoa produced by a primary 

 spermatocyte or to an egg with 

 its polar bodies, and the divi- 

 sions are considered as the first 

 and second maturation mitoses 

 (see p. Si). Three of the four 

 nuclei degenerate (d), the fourth 

 divides again. During this divi- 

 sion the granules of chromatin 

 contained in the nuclei separate 

 into two groups, one smaller 

 (Fig. 41, A, m.n.) than the 

 other (Fig. 41, A, f.n.). The 

 smaller nucleus might be con- 

 sidered comparable to the male 



nucleus, the other to the female. 



FIG. 41. Two views of the micro- 



The male nucleus migrates across nuclei during the conjugation of 

 the protoplasmic bridge between Pwmecium. A, the spindle formed 



during the division of the micronucleus 

 the tWO animals (Fig. 40, /) and which results in the production of a 





unites with the female nucleus of 



large female nucleus (f.n) and 

 smaller male nucleus (m.n). B, the 



one conjugant with the female nucleus 

 (f.n} of the other conjugant. (From 

 Calkins and Cull in Archiv f. Protist.) 



the Other Conjugant (Fig. 40, g\ fusion of the male nucleus (m.n) of 



Fig. 41, B), forming a fusion 

 nucleus (Fig. 40, h). Thus is 

 fertilization effected. 



The conjugants separate soon after fertilization (Fig, 40, g). 

 The macronucleus, which up to this time has remained at rest, 

 now assumes a vermiform shape, breaks up into small segments, 

 and then dissolves. Shortly after separation the fusion nucleus 

 of each conjugant divides by mitosis into two (i), these two into 

 four (j), and these four into eight nuclei equal in size (k). Four 



