140 THE TURBELLARIA. <§> 129. 



their chorion contains only loosely-arranged vitelline cells, among which 

 there is seen no trace of one or more germinative vesicles. The vitelline 

 cells always contain, beside a finely-granular albuminous substance, a round 

 nucleus which has a nucleolus. Both the nucleus and the granular sub- 

 stance are shifted from one side to the other of the cell by the very re- 

 markable peristaltic movements of the cell-membrane. After a time, these 

 movements cease, the cell-membrane disappears, and the contents mix with 

 those of other cells which have been affected in the same way : by these 

 means, little collections of vitelline substance here and there are formed, 

 which increase by the addition of other cells, — and finally are transformed 

 into roundish, nicely-defined embi"yos which become covered with ciliated 

 epithelium. From this time the embryos do not increase as before by th^e 

 external fusion of cells, but there is a muscular, discoid oesophagus formed 

 upon their periphery, and through this the remaining cells are ingested and 

 assimilated within the animal. 



Still later, the embryo, hitherto spherical, becomes flat and elongated at 

 two opposite points; — ultimately, and upon the appearance of the eye- 

 specks, it assumes exactly the form of the adult Planariae. 



The size of the young Planariae depends upon the number of embryos 

 developed in the same egg, for the smaller this number, the larger the 

 embrj'os at the time of their hatching, and vice versa. 



The cause regulating the number of embryos in an egg is yet un- 

 known.* ^^^ 



1 See my details upon this subject in the Bericht. The remarkable movements of the vitelline cells 



ueber die Verhandl. d. Berl. Akad. 18-41, p. 83. in the eggs of the P/anar/ae, and which I was the 



During the development of Planaria, one can, first to observe, have since been confirmed by A'o^- 



after a while, ascertain the number of vitelline cells liker, with Planaria lactea ; see Wie^mann''s 



assimilated by fusion and deglutition, by counting Arch. 1846, I. p. 291, Taf. X. I am unable to say 



their nuclei which are easily seen in the parenchy- whetlier or not the spontaneous movements observed 



ma of the body. According to Focke (loc. cit. p. by Qtiatre/ages (loc. cit. p. 169, PI. VII. fig. 6-9) 



201), the eye-specks, and the oesophagus are de- upon the larger portions of the vitellus of Po/^ceZis 



veloped very early in the young Mesostomum paWrf«s while in the oviducts, are of the same na- 



Ehrenbergii ; — a species with which each egg ture ; this naturalist himself supposes that these 



contains a single embryo only, and which is devel- portions were the embryos of this PlanariaA 

 oped while the egg is in the uterus. 



* [ End of § 129.] Kecent embryological studies doc. Strudelwiirmer, &c., p. IT ; hj Agassiz (^roc. 



have thrown some light upon this point — the Amer. Assoc. Advancem. of Sc. 2id meeting, 1849, 



alleged plurality of embryos in a single egg. The p. 438), who made the interesting observation that 



so-called egg in these cases is almost undoubtedly the Infusoria-genera, Kolpoda and Paramaecium, 



an ovarian sac. in which are developed many germs; are only larvse of P/anaria ; hy Girard (Ibid. p. 



some of these germs may perish, and the fewness 398), and by Muller {MiiUer's Arch. 1850, p. 



of those remainmg would give the appearance of 485). Muller has here some interesting remarks on 



an egg with many germs. — Ed. the relations of the study of these forms to the 



t [ § 129, note l.J The development of Plana- class Infusoria. —Ed. 

 ria has been also observed by Schmidt. Die Bhab- 



