192 



GENERAL EMBRYOLOGY 



ciously. However this may be it seems more instructive to classify the 

 process as karyogamic. 



Coming now to the consideration of typical karyogamic fusion we 

 find that all of the fertilization processes common to the Metazoa, as 

 well as those of most of the Protozoa, belong here. And here again 

 fertilization may be either endogamous or exogamous. 



Two general forms of karyogamy proper are usually distinguished, 

 although they are so clearly connected by transitional conditions that 

 they must be regarded as merely convenient groupings. These are 

 isogamy, where the pairing cells are similar in size, form and behavior; 

 and anisogamy, where the pairing cells are markedly dissimilar in size, 



FIG. 99. Autogamy in the Rhizopod, Entamceba histolytica. From Calkins, 

 "Protozoology," after Craig. A. Organism showing rods and granules of 

 chromatin in the nucleus, vacuole with some stained substance, and dense ecto- 

 plasm. B. Chromatin of the nucleus passing into the cytoplasm, as chromidia, 

 shown in C. D. Aggregation of chromidia to form secondary nuclei. E. 

 "Spore formation" by budding. F. Spores formed from buds. 



form, and behavior. That is, this distinction is not based upon differ- 

 ences in nuclear structure, or behavior during union, indeed, these are 

 essentially the same throughout karyogamy, but upon external characters 

 of the conjugating cells. 



Considering first isogamy, as the simpler and less modified process, 

 we find it restricted to the unicellular forms. Isogamic union frequently 

 occurs between two individuals of the usual vegetative type which do 

 not show, externally at least, any structural modifications usually 

 associated with gametic behavior (Fig. 100). This is most common 

 among the Flagellates, such as the familiar Copromonas, and Noctiluca, 

 but it occurs also in Actinophrys and in some species of Amceba. In other 

 cases the conjugating individuals show some modification in form as 



