140 THE TURBELLABRIA. 
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 embryos which become covered with ciliated 
epithelium. From this time the embryos do not increase as before by the 
external fusion of cells, but there is a muscular, discoid oesophagus formed 
upon their periphery, and through this the remaining cells are ingested and 
$ 129. 
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 
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. 
ueber die Verhandl. d. Berl. Akad. 1841, 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 cesophagus are de~ 
veloped very early in the young Mesostomum 
Ehrenbergii ;— a species with which each egg 
contains a single embryo only, and which is devel- 
oped while the egg is in the uterus. 
* [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. — Ep. 
+ [§ 129, note 1.] The development of Plana- 
ria has been also observed by Schmidt. Die Rhab- 
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 Kél- 
liker, with Planaria lactea; see Wiegmann’s 
Arch. 1846, I. p. 291, Taf. X. Iam unable to say 
whether or not the spontaneous movements observed 
by Quatrefages (loc. cit. p. 169, Pl. VIL. fig. 6-9) 
upon the larger portions of the vitellus of Polycelis 
pallidus while in the oviducts, are of the same na- 
ture ; this naturalist himself supposes that these 
portions were the embryos of this Planaria.t 
doc. Strudelwiirmer, &c., p.17; by Agassiz (Proc. 
Amer. Assoc. Advancem. of Sc. 29d meeting, 1849, 
p. 438), who made the interesting observation that 
the Infusoria-genera, Ko/poda and Paramaecium, 
are only larve of Planaria; by Girard (Ibid. p. 
398), and by Miller (Muller’s Arch. 1850, p. 
485). Muller has here some interesting remarks on 
the relations of the study of these forms to the 
class Infusoria. — Ep. 
