CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 423 
their position (cf. fig. 6) and become situated no longer ven- 
trally but laterally. When this change has been effected, 
their central cavity again communicates with the exterior by 
two channels, not this time by the side of the blastopore 
(mouth), but wholly lateral, viz. in the cephalic furrows. This 
secondary communication with the exterior is at the same time 
the definite one. The sacs have now become the lining of the 
cavity which is found in the posterior brain-lobes. These lobes 
in the adult are known as the “cephalic sacs,” as “the side 
organs,” etc., of the different authors. In a former publica- 
tion I have attempted to demonstrate that the cavity must be 
subservient to a curious direct respiratory process of the 
hemoglobiniferous nerve tissue. Embryology now renders it 
probable that they may at the same time have a sensiferous 
significance, as was the more generally accepted, and, in a 
certain sense, the current hypothesis. 
It is, indeed, almost impossible not to look upon the inner 
cavity of these (respiratory) sacs as clothed by a sensory 
epithelium, when we consider that this epithelium arises from 
the outer surface of the primary epiblast, from which in addi- 
tion only the epithelium of the proboscis takes its origin. That 
the latter is primarily of an eminently tactile significance was 
already noticed by me in an earlier volume of this journal 
(p. 349, 1883), when comparing Graff’s description of the 
origin of the proboscis in certain Rhabdoccels with the condi- 
tion of things in the Nemertea. The view is, moreover, 
strengthened by the very elaborate and most copious innerva- 
tion of the proboscis in all Nemertea, especially in those that 
are known to make the most constant use of this organ. 
The facts just recorded are all the more curious since I must 
now emphatically state that one of the results of my investi- 
gation is this, that no portion of the central nervous 
system of Lineus takesits origin in the epiblast either 
primary or secondary, but that the whole nervous system is of 
a mesoblastic origin. Those epithelia—in addition to that of 
1 ‘Zur Vergl. Anat. und Physiol. des Nervencyst. der Nemertinen ; Amst. 
Akad.,’ 1880. 
