222 INTRODUCTION TO CYTOLOGY 



generally looked upon as eggs historically: the maturation divisions 

 probably resulted formerly in a tetrad of eggs, whereas now only one 

 relatively large and highly differentiated egg is produced at the expense 

 of the other three cells, which remain small and functionless. 



Among the protozoa (see Minchin 1912) it has been found that in 

 those forms which appear to have their chromatin aggregated into no 

 definite number of chromosomes, there often occur two successive nuclear 

 divisions suggestive in certain respects of maturation divisions, a part of 

 the products then degenerating. Some have regarded this as a " casting 

 out of effete vegetative chromatin," an interpretation which was at one 

 time placed upon the maturation process generally. In many cases this 

 "reduction" of the chromatin occurs immediately prior to syngamy 

 (sexual union), 1 and so agrees with reduction in higher forms in taking 

 place at gametogenesis; but in other cases it immediately follows syngamy, 

 as in certain algae mentioned below. Other protozoa have been shown to 

 have a definite chromosome number which is regularly reduced in a man- 

 ner essentially comparable to that in the metazoa. 



In plants it is among the members of the lower groups (thallophytes) 

 that a striking diversity is shown in the stage of the life cycle at which 

 reduction takes place : in the groups above the thallophytes it is regularly 

 accomplished at sporogenesis. In the myxomycete, Ceratiomyxa, it 

 has been shown by Olive (1907) and Jahn (1908) that spore formation is 

 accompanied by a chromosome reduction. In the green algse it is in the 

 first two divisions of the zygote (either a zygospore or a fertilized egg) 

 that reduction occurs: this has been definitely established in Spirogyra 

 (Karsten 1908; Trondle 1911), Zygnema (Kurssanow 1911), Coleochcete 

 (Allen 1905c), and Cham (Oelkers 1916). In a number of other forms, 

 such as Ulothrix, (Edogonium, Sphceroplea, and Closterium, in which the 

 chromosomes are not well known, it is probable that the same condition 

 holds, since the zygote upon germination gives rise with considerable 

 regularity to four cells; in some cases ((Edogonium) these four cells are 

 zoospores. 



In the BROWN ALG^E Cutleria (Yamanouchi 1912), Zanardinia (Yama- 

 nouchi), and Edocarpus (Kylin 1918a) reduction occurs in connection 

 with zoospore formation. In Fucus, however, an exceptional condition 

 is found: here reduction takes place in the antheridium and oogonium 

 initials, in the first two divisions following the one delimiting the stalk 

 cell (Fig. 78, B). Since there are only three divisions in the oogonium, 

 which thus produces eight eggs, the eggs are but one division removed 

 from the four products of the maturation mitoses, a condition closely 

 approaching that in animals. That reduction in Fucus is associated 

 with gametogenesis was inferred by Strasburger (1897) and Farmer and 

 Williams (1898) and demonstrated by Yamanouchi (1909). 



1 See the cases of Actinophrys sol and Amceba albida, Chapter XII. 



