78 



ZOOLOGY 



SECT. 



daughter-colonies within the original cell- wall, by the rupture of 

 which they are finally liberated. In Volvox (Fig. 58), certain zooids, 

 called partJienogonidia (A, a), have specially assigned to them 

 the function of asexual reproduction : they divide by a process 

 resembling the segmentation of the egg in the higher animals 

 (D l -D 5 ), and form daughter-colonies which become detached and 

 swim freely in the interior of the mother-colony. 



A very interesting series of stages in sexual reproduction is 

 found in this group. In a number of cases copulation has been 

 found to occur between ordinary individuals without any special 

 differentiation of gametes. The union of the nuclei (karyogamy) 



H 



Fid. 58. Volvox glob a tor. 



. A, entire colony, enclosing several daughter-colonies; 



, the same during sexual maturity ; C, four zooids in optical section : D* D 5 , develop- 

 ment of parthenogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and 

 sperms ; H, oosperm ; a, parthenogonidia ; fl. flagellum ; ov. ovum ; ovy. ovaries pa 

 pigment spot ; spy. spermaries. (From Parker's Biology, after Cohn and Kirchner.) 



is always preceded in such cases by a reduction of their substance, 

 a process recalling the maturation of the ovum in higher animals 

 (p. 20). Copromonas (Fig. 57), which occurs in the faeces of frogs, 

 affords an example of this kind of copulation. Multiplication takes 

 place by binary longitudinal fission (a-d). Copulation also takes 

 place with reduction and karyogamy (e-f). This is not known to 

 recede any special form of multiplication, but the zygote or its 

 escendants by binary fission may pass into an encysted condition 

 (k) in which it is able to survive desiccation. 



p 

 d 



