M. Ussow’s Zoologico-Embryological Investigations. 211 
sections appears as a shallow but wide depression of the upper 
germ-lamella. At the time of its appearance the rudiment of 
the mantle is situated in the middle of the original germinal 
disk (centrum), with by far the greater part of it on the dorsal 
surface, whilst the somewhat elevated (constricting) part which 
subsequently grows round the ventral surface occupies only a 
very inconsiderable: space upon the latter. 
Above the mantle there are symmetrically on the two sides 
of the dorsal surface the two eye-ovals, and between them, at 
the boundary of the region of the arms, the above-mentioned 
rudiment of the buccal orifice. The lateral surfaces of the 
embryo represent the future cephalic lobes. 
On the following day, in all the Cephalopoda investigated by 
me, the branchie, the funnel, the arms, and the anal tubercle 
made their appearance. At the time when the rudiment of 
the mantle has become rather more constricted off from the 
blastoderm on the ventral surface, the cell-layer of the upper 
germ-lamella becomes somewhat thicker at the sides of the 
embryo (at first by longitudinal division, by which the cells 
are rendered higher, and then also by transverse division), 
and forms two inconsiderable prominences, which gradually 
grow and are the rudiments of the two so-called cephalic 
lobes. 
As regards the rudiments of the branchie, which are at first 
situated on the ventral side of the embryo not far from the 
margin of the mantle, these are developed from the more than 
one-layered thickening * of the dermo-muscular layer of the 
middle germ-lamella, which is covered by the cells of the 
upper lamella. 
On the boundary between the anterior cephalic lobe and the 
rudiment of the mantle a semilunar fold makes its appear- 
ance on each side of the embryo, produced by a thickening 
of the dermo-muscular layer, and covered, like all the organs 
mentioned, by cells of the upper lamella. This is the rudi- 
ment of the funnel, which consists of two halves, the margins 
of which coalesce very late, indeed only at the commencement 
of the third period T. 
Almost simultaneously with the appearance of the branchia 
there is formed between their pyriform rudiments, in the 
* At the end of the second and during the third period the cells in the 
middle of the solid branchial rudiments gradually become loosened, and 
tortuous reticulated ducts are produced in which the branchial arteries 
and veins with their numerous ramifications are formed (see Van Beneden, 
l.e. p. 9; Kolliker, 7. ce. p. 89; Metschnikoff, 7. c. p. 61). 
+ On the dorsal surface the two halves of the funnel approach each 
other as early as the fourth day of the second period, 
