38 FIRST GROUP.— THALLOPHYTES. 



life-long ; but here the disk begins to deepen into a bowl, the edges draw together, close 

 up and form a hollow sphere, which, as was said, consists of sixteen or thirty-two cells. 

 When the several cells have developed their cilia, the young colony begins to move, and 

 being set free by the dissolution of the envelope of the mother-colony, is able to produce 

 new daughter-colonies in the evening of the day on which it was itself formed. Thus the 

 coenobia have two distinct poles, one on which the four originally middle cells lie, and 

 one where the edges of the disk unite in the formation of the sphere. 



The sexual organs are dioeciously distributed, and the colonies are therefore male and 

 female. The cells of the latter are only slightly distinguished from the vegetative cells- 

 The cell-walls swell up, the several cells of the female colony, the oospheres, are thereby 

 separated from each other, and are rather larger than the vegetative cells. The male 

 colonies produce sper/natozoids, the formation of which begins in the same manner as that 

 of the asexually produced daughter-colonies. But in the case of the spermatozoids the 

 daughter-cells which result from the division of a cell are disposed in the same plane 

 (Fig. 18, IV) ; each cell becomes elongated, developes a red spot laterally near its anterior 

 extremity and two cilia at the same extremity, and becomes a spermatozoid in a way 

 which will be made clear by comparing Fig. 18, / and ///- F/, and the explanation 

 appended. The colour at the same time changes from green to yellow. The sperma- 

 tozoids collected into a bundle continue in movement inside the cell in which they 

 were formed, then escape from it and swim about in freedom as a male colony. If they 

 fall in with a female colony, the cilia of the two become entangled together, the male 

 colony is caught and held, and then the bundle of spermatozoids falls apart (see Fig. 18, 

 M-i, Mo, M.), and the spermatozoids, now isolated, elongate still more and force their 

 way through the gelatinous envelope of the female colony. When they reach the 

 oospheres, they creep and grope about among them and attach themselves to them in 

 numbers, till at length one spermatozoid mingles its substance with that of an 

 oosphere, whereupon the latter as an oospore invests itself with two coats. The 

 chlorophyll assumes the well-known brick-red colour, which is characteristic of the 

 resting stage of the Chlorophyceae. The germination of the oospores, which takes 

 place in the spring after their formation, has not been observed either by Goroshankin 

 or by myself. We may assume, from our knowledge of Volvox, that a eudorina- 

 colony is produced by each germinating oospore, and with a similar process of 

 division to that displayed in the formation of the asexual colonies. 



The course of development of the genus Volvox^ itself, which agrees in general with 

 that of Eudon'jui, may be briefly touched upon in this place. The coenobium consists 

 of a much larger number of cells, the single volvox-spheres, even those of V. minor, 

 being plainly visible to the naked eye. In the asexual colonies of V. globator, only 

 a few of the cells, eight in number, distinguished at all times from the rest by their size, 

 are capable of producing daughter-coenobia. The mode of their formation is as in 

 Eudorina. The distribution of the sexual organs is monoecious or dioecious, but the 

 sexual reproductive cells are always few in number among the many sterile cells of 

 a coenobium. The bundle of spermatozoids is formed in the same way as in Eudo7-ina ; 

 32-64 spermatozoids proceed from one mother-cell. In shape, too, they are like those of 

 Eudorina, but the cilia, according to Cohn, are inserted laterally, as in Fucus. The 

 germination of the oospores is known only in V. mi7tor. The outer coat of the spore 

 bursts, and the protoplasm emerges surrounded by the swollen inner membrane. It 

 divides at first into an eight-celled disk, which is transformed into a hollow sphere, the 

 coenobium, in the manner described above in the case of Eudorina. 



In the genera Gonium and Stephanosphaera - we may assume, with a high degree 

 of probability, that the process of fertilisation is the same as in Pandorina, viz. by 

 means of motile gametes of similar form. 



' [Drude, Ueber Bau n. Entw. d. Kugelalgen Volvox Abh. d. naturw. Ges. z. Dresden 1882"). 

 ^ Hieronymus, G., Ueber Stephanosphaera pluvialis, Cohn (Cohn's Beitr. z. Biol. d. Pflanz. 

 Vol. IV).] 



