CYTOLOGY OF REPRODUCTION IN ANIMALS 



133 



The neuromotor apparatus is an unusually interesting specialization in 

 manj^ protozoan and flagellate cells. In some it is absent; in others it 

 consists of a flagellum, a basal granule {blepharoplasi) , and a strand 

 (rhizoplast) connecting the latter with a centrosome at the nucleus 

 (Fig. 98). Sometimes a single body acts as both centrosome and bleph- 

 aroplast. In more elaborate forms there is a central mass, the motoriuni, 

 from which strands run out to the several swimming oi-gans. In Ciliata 

 there are also numerous ^'ery fine strands (the "siher-line system") run- 

 ning beneath the rows of cilia and near the stinging organs, or trichocysts. 



Digestion in the more highly differentiated Protozoa may occur in a 

 special tract running from mouth to anus. Special contractile and 

 supporting structures also occur (Fig. 9). Far from being simple, such 

 protozoans are the most complicated single cells known. 



Fig. 98.— 

 A flagellate 

 [M enoidium) . 

 Explanation in 

 text. [After R. 

 F. Hall.) 



Fig. 99. — Mitosis during fission in a protozoan (Barbulanympha). 

 Longitudinal section of anterior end of cell. The two large elongate 

 centrioles are connected anteriorly with the two flagellated areas and 

 posteriorly with the achromatic figure. Some of the astral rays con- 

 nect at the nuclear membrane with intranuclear chromosomal fibers. 

 (After L. R. Cleveland.) 



The asexual reproduction of a protozoan by fission may appear to be a 

 simple process externally, but in the more highly organized species it 

 involves a very complicated series of changes. Not only does the 

 nucleus divide mitotically, but the components of the motor apparatus 

 and the other specialized regions are also doubled variousl}^ bj- division 

 and ne\^ formation. A prominent part in the process is usually played 

 bj'- the centrosomes or blepharoplasts (Fig. 99). 



Sexual reproduction in these small animals involves nuclear divisions 

 and fusions which in manj^ cases have not been fully interpreted. In 

 some genera, however, the details of the process have been made out 

 sufficiently well to show that in certain fundamentals it corresponds to 

 the meiosis-syngamy cycle seen in Metazoa. These changes may occur 

 entirely within one individual cell (autogamy) . Syngamy involving two 

 individuals may include the complete fusion of morphologically similar 

 or dissimilar gametes, or only a mutual exchange of nuclei between 

 individuals in temporary conjugation (Fig. 100). In Paramecium cauda- 

 tum the latter type of process occurs as follows: 



