FERTILIZATION 



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acters. Several observations, however, fail to harmonize with this view. 

 In some echinoderms the middle piece, which contains the chondriosomal 

 material, is not distributed to the daughter cells during the cleavage 

 divisions, but remains in one of them. It is unlikely that hereditary 

 material would behave in this manner. Furthermore, in the worm, 

 Nereis, the middle piece does not even enter the egg. In Peripatus 

 (Montgomery 1912) the chondriosomal mass is thrown out of the 

 spermatid. Wildman (1913) points out that in Ascaris also the chon- 

 driosomes may be largely lost during spermatogenesis. Although their 

 almost constant presence in the spermatozoon indicates that they have 

 a special physiological role comparable to that in other cells, there is as 

 yet no adequate reason to regard them as important in the transmission 

 of the factors of inheritance. 



FIG. 110. Chromidiogamy in Arcella. (From Minchin, after Swarczewsky.) 

 Somewhat modified. 



Fertilization in Protozoa. Among the protozoa there are found 

 several modes of sexual union very different from that described above for 

 the higher animals. Four illustrative cases may be cited (see Minchin 

 1912). 



In Arcella, a rhizopod with a hemispherical shell, the protoplast has 

 two "primary nuclei" and many scattered chromidia. Two individuals 

 come together with their shell openings opposite one another (Fig. 110), 

 and the protoplasm of one flows almost entirely into the other shell, 

 where it mingles with that of the other individual. The primary nuclei 

 degenerate, while the chromidia become divided up into fine granules. 

 The protoplasm with the fine chromidia now becomes evenly distributed 

 in the two shells, after which the latter separate. In each individual the 

 chromidia now form "secondary nuclei;" around these are organized 

 uninucleate amcebulse which escape from the shell and give rise to new 

 Arcella individuals. This process, in which the chromatin chiefly con- 



