322 EMBRYONIC DEVELOPMENT 



October 1848. — The eggs exist as organic cells, similar in every respect to the 

 cells constituting the organic tissues- at large. 



December 1S48. — 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 

 (tigs. 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 eggreassumes its primitive 

 appearance (fig. 46). 



May 27 — 31. — State of repose, preparing the embryo properly so called (figs. 

 17—50). 



June ]. — 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 vibrillfe now appear (figs. 55 and 56). 



June 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). 



June 6. — Escaping of the embryos from the eggs as larvfe. The latter are polymor- 

 j)hic 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 Planarise re- 

 semble infusoria of the genera Kolpoda (figs. 94 — 103) and even Paramecium 

 (figs. 57—65). 



June 14 — 16. — The lively larvse 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. 



The egg of Planocera elliptica passes through a series of successive periods, each of 

 which may be thus characterized : 



