214 M. Ussow's Zoologico-Emhryological Investigations. 



pigment-granules are jiroduced upon tlic surface of the 

 retina *. 



The two cephalic lobes, which have now become consider- 

 ably thicker, rise more and more abin-e the nutritive vitellus ; 

 whilst the region of the arms, situated upon the ccpiator, becomes 

 constricted, and thus forms a luirrower boundary between the 

 embryo and the spherical yelk-sae. The embryo, when ex- 

 amined from either the ventral or dorsal side, has a lyriform 

 shape ; its lower part (mantle) is considerably scjiarated by con- 

 striction from the ventral surface, the middle jjartf tolerably 

 broad ; and the region of the arms forms a very noticeable notch 

 between the yelk-sac and the true embiyo. 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 a])pearance of two spherical masses of cells 

 of the intestino-librous layer of the middle lamella, which are 

 situated on the ventral side of the embryo, near the sides of the 

 branchife, 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 hearty 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- 

 tions}:. 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 intcstino-fibrous layer, which is at first s})herical 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 ciliarc produced from tho 

 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 yh-t/omiutd, Liiliyo, Sec. tlie lens consists of 

 concentric layers of a structureless transparent substance. In tlie third 

 period the retina, consisting of two layers of cylindrical cells, -which was 

 at first convex, sinks and becomes semilunarly concave ; the dark browu 

 pigment, singularly enough, persists upon the surface of the retina until 

 the close of embryonal life. 



t From this part are gradually fomied the head, all the organs enclosed 

 within it, and some organs of the trunk. 



X The very large pericardial cavity is very distinctly perceptible in 

 the first half of the third period. 



§ It is particularly distinct in sections of tho first half of the tliird 

 period, in the form of an oval aggregation of cells. Tlie cavity, embraced 

 by thin walls, is very slowly formed; so thai the development of the 

 auricles considerably precede.-^ that of the ventricle. 



