OF PLANOCERA ELLIPTICA. 
23 
were really a normal state of existence in the embryogeny of Planarians, or an ab- 
normal one, in consequence of the limited space in which the larvae had been kept. 
But soon afterwards, having had an opportunity to observe the same bodies out at sea, 
in their natural element, there could be no hesitation in regarding this chrysalis state 
as a natural period in the genetic development of Planocera elliptica. 
How long this period will last, is a question now to be investigated ; also, whether 
the perfect animal is the next step, or whether there are other stages of development, 
or other metamorphoses. 
XIV. 
CHRONOLOGICAL SKETCH. 
In 1849, a chronological narrative of the development of Planocera elliptica was read 
before the American Association for the Advancement of Science,* and subsequently 
before the Society of Natural Sciences of Neuehatel, Switzerland.! 
October 1848. — The eggs exist as organic cells, similar in every respect to the 
cells constituting the organic tissues at large. 
December 1848. — From simple primordial cells, the eggs have grown to a semi- 
transparent sphere, containing some vitelline substance, a germinative vesicle and a 
germinative spot. The eggs can no longer be mistaken for primordial organic cells 
(figs. 1—6). 
May 1849. — The eggs, still within the maternal body, have become opaque by the 
increase of the vitelline substance, which conceals now entirely the germinative 
vesicle, visible only upon the compression of the egg (figs. 5 and 6). 
May 25, A. M. — Laying of the eggs. These consist of an external envelope, 
a zone of albumen and an opaque vitellus, occasionally showing a central clear space 
indicating that the germinative vesicle is still present. The germinative spot, however, 
has already disappeared (figs. 12 and 13). 
May 25 P. M.— Commencement of the phase of the division of the yolk (fig. 15). 
May 26 P. M. — That division comes to a close. The egg reassumes its primitive 
appearance (fig. 46). 
May 27 — 31.— State of repose, preparing the embryo properly so called (figs. 
47—50). 
June 1 .—The yolk or embryo begins to revolve, and will keep in motion until it 
shall be transformed into a chrysalis (figs. 52 and 53). 
June 2. — Large transparent cells are formed in the centre of the embryo, still 
spherical in form, whilst upon its surface vibrillse now appear (figs. 55 and 56). 
*Proc. Amer. Assoc. Adv. Sc. second meeting, held at Cambridge, in August 1849. Cambridge 1850, 398. 
fBull. de la Soc. des Sc. Nat. de Neuch. II. 1850, 300. 
4 
