600 FERTILIZATION 



heterogamy is seen in Actinophrys sol. When the gametes unite, one of 

 them sends out a pseudopodial process to the other, to initiate the fusion. 

 This pseudopodium is formed by only one member of the pair, and the 

 maturation processes in this one seem to occur a httle ahead of those in 

 the other. These slight differences between the two gametes are inter- 

 preted as the beginnings of differentiation toward maleness and female- 

 ness. In rare cases the pseudopodium of the male fails to make contact 

 with the female, and then the female sends out a pseudopodium which 

 brings about fusion. The indication here is that whatever the degree of 

 differentiation of the gametes is, this differentiation is reversible. Perhaps 

 the potentiality for pseudopodial formation is retained in all gametes, but 

 only the one completing maturation first ordinarily exhibits it. When 

 neither gamete succeeds in connecting with its pseudopodium, no sexual 

 differentiation is demonstrable. In such cases both gametes form par- 

 thenogenetic cysts. 



The most noteworthy phase of gamete formation in Actinophrys sol 

 is the striking similarity of the meiotic stages to those of the Metazoa. 

 Following the progamous division of the gamont into the two gameto- 

 cytes, two maturation divisions occur which reduce the chromosome 

 number from the diploid forty-four to the haploid twenty-two. In the 

 prophase of the first maturation division, the chromatin forms into 

 slender looping threads (leptonema) which pair off (parasynapsis), 

 become thicker (pachynema), and are obviously twisted around each 

 other (strepsinema). Then they shorten (diakinesis) into compact 

 chromosomes on the metaphase spindle, and the two parts of the bi- 

 valent chromosomes separate in the anaphase, twenty-two univalent 

 chromosomes going to each pole. One product of this division degen- 

 erates, and the other undergoes the second maturation division, which 

 is equational. The twenty-two chromosomes split longitudinally, so that 

 the pronucleus and the two polar bodies of each gamete have twenty- 

 two chromosomes. 



It seems that this relatively simple heliozoon has developed a matura- 

 tion process that is as highly specialized and clear-cut as any found in 

 the Metazoa. It is probably safe to say that further diligent search will 

 undoubtedly reveal other species of Protozoa with equally well developed 

 meiotic phenomena. 



