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have referred to or transcribed Hatschek’s work appear to 
imagine that he described the mesoderm as originating as a 
series of pairs of independent pouches, which is not the case. 
According to Hatschek the lateral folds, whilst still in open 
communication with tlie gut, are divided by constrictions 
into four or five “ somites ” on each side, of Avhich, however, 
only the first pair are completely separated from the rest, 
the others constituting a single moniliform fold on each 
side. A little later the front part of this moniliform fold 
becomes completely separated from the gut, and its consti- 
tuent somites then become completely separated from one 
another, the last only retaining its communication with the gut. 
As new somites are formed the animal grows in length and 
the primary fold is prolonged backwards; from this fold these 
somites are cut off, and only the last pair at any moment is in 
open communication with the gut. The first pair of somites, 
however, which were early separated from the rest, long 
retain their communications with the gut. Each of the somites 
undergoes segnnentation into a dorsal portion which consti- 
tutes the myotome, and a ventral portion, which fuses with 
its predecessor and successor to form the splanchnoccele, the 
right and left splanchnoceles fusing to form the general 
abdominal cavity of the adult. From the most anterior part 
of the gut two diverticula grow out which become cut off 
from it; these are termed by Hatschek “ head-cavities”; of 
these the left becomes a small thick-walled vesicle, which sub- 
sequently acquires an opening to the exterior, whilst the right 
becomes a large thin-walled vesicle which forms the cavity of 
the snout in the young Amphioxus. It is evident that most 
of Hatschek’s observations are to be regarded as extensions of 
Kowalevsky’s results, due to his more perfect method of 
technique. The only point in which they radically differ is 
in the position they assign to the blastopore, and in this 
matter Hatschek frankly acknowledged that his conclusions 
were far from certain. 
So clear and simple an account as that given by Hatschek 
remained for a long time in favour, but in 1894 it was attacked 
