214 M. Ussow’s Zoologico-Embryological Investigations. 
pigment-granules are produced upon the surface of the 
retina *. 
The two cephalic lobes, which have now become consider- 
ably thicker, rise more and more above the nutritive vitellus ; 
whilst the region of the arms, situated upon the equator, becomes 
constricted, and thus forms a narrower boundary between the 
embryo and the spherical yelk-sac. The embryo, when ex- 
amined from either the ventral or dorsal side, has a lyriform 
shape ; its lower part (mantle) is considerably separated by con- 
striction from the ventral surface, the middle part} tolerably 
broad ; and the region of the arms forms a very noticeable notch 
between the yelk-sac and the true embryo. The nutritive 
vitellus enclosed within the embryo has the form of a hemi- 
sphere with a tuberculiform process which penetrates into the 
mantle on the dorsal surface. 
The fourth day of the second period of development is cha- 
racterized by the appearance of two spherical masses of cells 
of the intestino-fibrous layer of the middle lamella, which are 
situated on the ventral side of the embryo, near the sides of the 
branchiz, at the apices of which two prominences are produced 
at this time. These solid aggregations of cells form the rudi- 
ment of the auricles of the heart, which are afterwards sur- 
rounded by a pericardium. The pericardium consists of a cell- 
layer of the upper lamella, which penetrates between the 
mantle and the funnel, and completely clothes the aggrega- 
tionsf. The rudiment of the ventricle of the heart lies between 
the rudiments of the auricles, can only be detected with diffi- 
culty§, and consists of a solid aggregation of cells belonging 
to the intestino-fibrous layer, which is at first spherical but 
afterwards cylindrical. By degrees the cells separate from 
* With respect to the development of the organs of vision, I must add 
that the lens is formed at the beginning of the third period as a fluid, 
gradually hardening secretion of the corpus ciliare produced from the 
above-mentioned fold. Its form changes pretty rapidly from cylindrical 
to oval, and finally becomes spherical. In longitudinal and transverse 
sections of the embryos of Argonauta, Loligo, &c. the lens consists of 
concentric layers of a structureless transparent substance. In the third 
period the retina, consisting of two layers of cylindrical cells, which was 
at first convex, sinks and becomes semilunarly concave ; the dark brown 
pigment, singularly enough, persists upon the surface of the retina until 
the close of embryonal life. 
+ From this part are gradually formed the head, all the organs enclosed 
within it, and some organs of the trunk. 
{ The very large pericardial cavity is very distinctly perceptible in 
the first half of the third period. 
§ It is particularly distinct in sections of the first half of the third 
period, in the form of an oval aggregation of cells. The cavity, embraced 
by thin walls, is very slowly formed; so that the development of the 
auricles considerably precedes that of the ventricle. 
