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DE. E. VON WILLEMOES-SUBM ON THE 
yelk-cells, only in the middle some small bodies may be seen reflecting the light Very 
strongly, and looking like minute granules of fat (Plate 10. fig. 4). 
About the formation of the embryo there are, or at least there were, two opinions, 
the one backed by Fritz Muller, and for some time (according to Van Beneden) also 
by Claus, maintaining that a total segmentation takes place, and that the embryos are 
developed in full without a preceding “ Primitivstreifen,” which means a larger gathering 
of cells on the ventral side of the embryo*, while Van Beneden has seen and figured 
the latter in Sacculina. 
Buchholz mentions, like Van Beneden, the formation of the blastoderm, but does 
not seem to have seen a primitive streak. According to him one of the two halves, 
which are the result of the first segmentation, divides again, and its segments overgrow 
the other half, forming a blastodermic cuticle round it, in which grooves may soon be 
seen, indicating the future position of the three appendages. 
Lepas fascicularis is not a very favourable object for such an inquiry; nevertheless 
I have arrived at a result which holds somewhat an intermediate place between Van 
Beneden’s and Buchholz’s observations. I saw the blastoderm forming very much in 
the way described by the former, but I was unable to find any definite trace of a primi- 
tive streak. The first alteration which takes place is the formation of two segments 
(fig. 5, where a small third one may be seen between the two), which are unequal 
in size, whereupon the lower and larger one divides again (fig. 6). In these stages 
of segmentation you see already two to four large, transparent, nucleated cells, which 
separate themselves from the yelk-globules in the middle of the ovum, and the 
number increases very much in the next stage (fig. 7). We find first eight and then 
twelve. In fig. 7, y, x., I have given a sketch of two sorts of cells which are 
now contained in the ovum, and which, after breaking its walls, can be separately 
inspected : these are large blastodermic cells with a single nucleus, with only a few 
granules, and small, very granular yelk-globules. Also in Van Beneden’s drawing I 
do not find a stage between the mulberry stage and the stages of first segmentation ; 
and I must confess that in Lepas fascicularis I never could find but indistinct 
traces of the later stages of segmentation. The large cells seem to break out and 
include the rest of the yelk, thus forming a blastoderm (fig. 8) consisting of small 
nucleated cells. This was well enough to be seen with reflected light ; but the interior 
of the ovum is so little transparent that I could not make out whether there is a primi- 
tive streak on one side or not. The blastoderm now loses its cellular look, and has 
the appearance of a granular cuticle, just as it has been seen by Buchholz in Balanus ; 
and then a groove, which was already visible in the last two stages, is seen to become 
deeper, and on each side of it the first traces of the three pairs of appendages (fig. 9, 
a and b) become visible. Very soon the labrum also appears; the tail is differentiated, 
and setse may be seen at the top of the foot-joints (fig. 10). In Sacculina the embryo 
throws the egg-shell off before this happens, and is instead (according to Van Beneden) 
* Fritz Muller, ‘ Fiir Darwin,’ 1864, p. 64. 
