CYTOLOGY OF REPRODUCTION IN ANIMALS 131 



do not develop so well as the diploid zj-gotes. Moreover, when the 

 parthenogenetic development of monoploid animal eggs is induced by 

 artificial means, the few individuals that are successful in developing 

 through metamorphosis have almost invariably become diploid. In the; 

 mitoses occurring in parthenogenetic frog embryos, for instance, both the 

 monoploid and the diploid numbers are found, indicating a gradual 

 doubling jjrocess. Evidently the embryos not undergoing this change 

 fail to survive. Among salamanders obtained from fertilized eggs 

 subjected to low temperatures there are some that are monoploid at 

 least up to the stage at which ovaries with oocytes are developed. 



Development in the monoploid condition is therefore a possibility in 

 animals as it is in plants, provided the conditions favoring it are present. 

 As a rule monoploid animals derived by such means are almost if not 

 completely sterile. Among natural parthenotes, however, fertility may 

 obtain. In the male honey bee, for example, functional sperms are 

 produced after a single spermatocyte division which is equational in 

 character. All these facts indicate that in the evolution of various groups 

 of organisms there have been adjustments of the reproductive process 

 to a considerable range of variations in the nuclear cycle. This is further 

 evident in plants, most of which have reproductive cycles differing 

 widely from those in animals. 



Protozoa. — The Protozoa are a very large group of very small organ- 

 isms. They are so diversified in structure and type of life cycle that long 

 chapters would be required to describe their cytology fully; hence this 

 discussion must be restricted to a few representative features of special 

 interest. 



Protozoa are characteristically unicellular, i.e., the body has the 

 general structure of a typical protoplast, with cji^oplasm, membranes, 

 and one nucleus. Some protozoologists prefer to regard them as non- 

 cellular, meaning that they are not subdivided into compartments but 

 simply have the structure necessary to a small but complete mass of 

 protoplasm. A tendency to become coenocj^tic or multicellular does 

 appear here and there in these animals, but evolution within the group 

 has proceeded mainly along other lines. It is customary to say that 

 Protozoa are small because they are unicellular, but as we compare 

 their frequently elaborate structure and behavior ^^^th what is seen in 

 Metazoa something may be gained from the concept that they arc 

 unicellular because they are small. Effective differentiation in very small 

 masses can occur without a multiplicity of nuclei and cells. Cell division 

 serves them in reproduction, but not in the building of the individual 

 body. 



Nuclear conditions vary widely in the Protozoa. Most protozoans 

 have a single nucleus, but probably in all groups the binucleate and 



