140 



THE TÜRBELLARIA. 



^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, around 

 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 afi'ected 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 embryos 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 anhnal. 



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 Pla^iariae. 



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 

 embryos 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, 

 ueljer die Verhandl. d. Berl. Akad. 18«, p. 83. 

 During the development of Planaria, one can, 

 after a while, ascertain the number of vitelline cells 

 assimilated by fusion and deglutition, by counting 

 their nuclei which are easily seen in the parenchy- 

 ma of the body. According to Focke (loc. cit. p. 

 201), the eye-specks, and the oesophagus are de- 

 veloped very early in the young Mesostomum 

 EhrenbeTgU ; — a species with which each egg 

 contains a single emln'yo only, and which is devel- 

 oped while the egg is in the uterus. 



The remarkable movements of the vitelline cells 

 in the eggs of the Planariae, and which I was the 

 first to observe, have since been confirmed by Kol- 

 liker, with Planaria lactea ; see fVief^mann^s 

 Arch. 1846, I. p. 291, Taf. X. I am unable to say 

 whether or not the spontaneous movements observed 

 by quatrefages (loc. cit. p. 169, PI. VII. fig. 6-9) 

 upon the larger portions of the vitellus of Polycelis 

 pallidas while in the oviducts, are of the same na- 

 ture ; this naturalist himself supposes that these 

 portions were the eml)ryos of this Planaria.^ 



* [ End of ^ 129.] Recent embryological studies 

 have thrown some light upon this point — the 

 alleged plurality of embryos in a- single egg. The 

 so-called egg in these cases is almost undoubtedly 

 an ovarian sac, in which are developed many germs; 

 some of these germs may perish, and the fewness 

 of those remaining would give the appearance of 

 an egg with many germs. — Ed. 



t [ § 129, note 1.] The development of Planfl- 

 ria has been also observed by Schmidt. Die Rhab- 



doc. Strudelwürmer, &c., p. lY; by Agassiz (Proc. 

 Amer. Assoc. Advancem. of Sc. 'i"<i meeting, 1S49, 

 p. 438), who made the interesting observation that 

 the Infusoria-genera, Kolpoda and Paramaecium, 

 are only larvaj of Planaria ; by Girard (Ibid. p. 

 398), and by Müller (Müller^s Arch. 1850, p. 

 485). Müller ha.s here some interesting remarks on 

 the relations of the study of these forms to the 

 cli-ss Infusoria. — Ed. 



