190 



GENERAL EMBRYOLOGY 



divide several times forming a considerable number of smaller nuclei; 

 finally the cytoplasm divides correspondingly, producing thus a group of 

 zoospores (brood formation). Such processes as these occur most 

 typically in the Mycetozoa (Myxomycetes) and also in such forms as 

 Arcella, Actinophrys, and some Foraminifera (Fig. 97). 



Finally we come to a third general form of conjugation known as 

 karyogamy, which as the word indicates involves primarily a process of 

 nuclear fusion of the conjugating cells, although accompanied, of course, 

 by cytoplasmic fusion which may be of hardly secondary importance. 

 For instance, in some of the Infusoria the achromatic spindles fuse, as 

 well as the nuclei and undifferentiated parts of the cytoplasm; in some 



FIG. 97. Plastogamy in the Rhizopod, Arcella vulgaris. After Elpatiewsky. 

 A. Plastogamic union of about five individuals, apparently preparatory to the 

 formation of zoospores ("pseudopodiospores"). B. Reproduction (formation 

 of " macroamosbae ") following plastogamy. c, chromidia; n, nuclei; d, degen- 

 erating nuclei, 



of the lower plants, even the plastids of the gametes, perhaps also their 

 centrosomes, fuse together during conjugation. 



Most of the more familiar fertilization processes of the Protozoa are 

 essentially karyogamic, but, as a rule (as in the Metazoa), not all of the 

 nuclear substance of the cell is involved in the process. For usually, 

 as a preliminary to conjugation, the vegetative nucleus gives off, into the 

 cytoplasm, portions of its substance (chromidia) . These may be formed 

 as a result of general nuclear disintegration, or the nucleus may remain 

 quite intact and extrude chromidia, either directly through its mem- 

 brane, or by a process of nuclear budding. Some of these chromidia 

 are concerned in reproduction ; such are termed idiochromidia. Karyo- 

 gamy, consequently, involves only a portion of the nuclear substance 

 ordinarily, and the remaining chromidia and vegetative nuclear struc- 

 tures may even break down and disappear during the process of ferti- 

 lization. Altogether these processes of chromidia formation are diverse 



