BEPRODUCTION BY GERM-CELLS 269 



is found, among others, in the Tolvocineae, those green, spherical, fresh- 

 water cell-colonies which we have already studied in relation to re- 

 production by asexual germ-cells. Among them it is the rule that, 

 after a long series of generations producing only ' asexual ' germ-cells, 

 colonies occur in which each germ-cell is no longer able to develop 

 a new colony alone, but can do so only after it has united with 

 another germ-cell. 



Xow, as we have seen, there are Volvocinese in which the 

 differentiation of cells into those of the body (soma) and those 

 concerned with reproduction has not been established, and all the 

 ceUs are therefore alike. In these, as for instance in the genus 

 Pandorina (Fig. 62, p. 257), when sexual reproduction is to occur 

 the whole colony breaks up into sixteen cells ; these burst forth from 

 the gelatinous matrix in which they have been hitherto enclosed, 

 swim about in the water with the help of their two flagella, meet 

 other similar free-swimming ceUs and conjugate with these. The two 

 swimming cells come close to each other, draw in their flagella, sink 

 to the ground in consequence, and fuse completely both as to the cell- 

 body and the nucleus. They assume a spherical form, lose the ej'e- 

 spot, become surrounded with a tough cell-skin or cyst, and so remain 

 for a longer or shorter time as so-eaUed 'zygotes' or lasting spores. 

 Then they develop by repeated cell-division into one of the sixteen- 

 celled PaTidorina colonies with which we are already familiar; this 

 bursts forth from the capsule and swims freely about in the water 

 again. 



Here, therefore, the so-eaUed sexual reproduction depends on the 

 fusion of two cells similar in appearance, and when this phenomenon 

 was first known it was regarded as something quite different from the 

 corresponding reproduction in other multicellular organisms. But we 

 now know that quite nearly related Volvoeinese belonging to the 

 genus Volvox and to other genera, which exhibit a differentiation 

 into body-cells and reproductive cells, may reproduce sexually by 

 means of two different kinds of germ-cells ; and we have also learned 

 through Goebel and others that even genera like Pandorina, which 

 consist of only one kind of cells, may yet produce male and female 

 reproductive cells differing essentisJly in form from one another. In 

 Eudonna, for instance, a gelatinous sphere containing sixteen or 

 thirty-two individual ceUs, asexual reproduction occurs in exactly the 

 same way as in Pandorina, that is, each of these cells divides four 

 or five times in rapid succession, and thus forms a new colony, which 

 then bursts forth ; but when the time for sexual reproduction comes 

 the colonies behave differently, for some become female and some 



