24 
EMBRYONIC DEVELOPMENT 
Juno 3 — 5. — The embryonic sphere loses its primitive form by the flattening of one 
of its poles. From this flat surface several cones rise up. between which a groove 
is formed, which opens the sphere. The little creature is now symmetrical (figs. 75 
—93). 
J une 6. — Escaping of the embryos from the eggs as larvae. The latter are polymor- 
phic and endowed with a plastic elasticity by means of which they pass from one 
shape into another. At this stage of their development the larvae of Planariae re- 
semble infusoria of the genera Kolpoda (figs. 94 — 103) and even Paramecium 
(figs. 57 — 65). 
June 14 — 16. — The lively larvae transform into an immoveable chrysalis-like body 
(figs. 104 and 105). 
The further progress of development I have not yet been able to trace out. 
XV. 
RECAPITULATION. 
§> 1. The egg of Planocera elliptica passes through a series of successive periods, 
each of which may be thus characterized : 
I. Period of evolution. This period embraces the time which elapses between 
the origin of the egg and the laying. The egg originates like the organic cells. It 
generates its own mass around a new centre of attraction until it becomes capable of 
leading an independent life. During this period the egg is dependent upon the 
mother, and has its growth. 
II. Period of preparation. Embraces the time wdiich elapses from the laying of 
the egg to the first manifestation of the embryo. The egg is full grown ; the vitelline 
mass divides, works and kneads its substance, and prepares it for the future being. 
During this period the vitellns is in a state of rest, in other words, immoveable, but is 
no longer connected with its parent. 
III. Period of the embryo. Embracing the time which elapses between the first 
manifestation of the embryo and its escaping from the egg envelope. The vitelline 
sphere is in motion and assumes various shapes. 
IV. Period of the larva. Lasts from the escaping of the embryo from the egg, 
to its transformation into a mummy-like body. The young animal moves freely 
about in the surrounding medium, still assuming divers shapes. 
V. Period of the chrysalis. It commences ivhen the lively larva, losing its 
polymorphic shape, becomes an immoveable body, simple and uniform in external 
appearance. The end of this period is not known. 
