296 
SIDNEY F. HARHER. 
free swimming existence, the brain has the appearance of 
fig. 50 and fig. 51, two consecutive sections (horizontal) of the 
same embryo. Fig. 51 shows the two eyes (o) which the 
neighbouring section (fig. 50) has just missed, although it 
passes through the fibrous region of the brain at its thickest 
part ( fbr .) On the outer side alone of the fibrous portion is 
a mass of ganglion-cells, which in agreement with the mode of 
their development are not arranged in a single layer. There 
thus exists a well-marked distinction between the development 
of the adult ganglion (suboesophageal) and that of the larval 
brain. The latter is formed by a process of invagination, the 
fibrous layer making its appearance entirely on the deep side 
of the ganglion- cells, which are arranged in more than a single 
layer ; the former arises by the differentiation of a solid cell 
mass from a thickened ectodermic region, the cells which com- 
pose the ganglion separating from one another centrally, and 
depositing a fibrous layer on their inner sides in such a 
manner that the adult organ consists of a fibrous core sur- 
rounded by a single layer of cells. 
The finer histological details of the structure of the brain 
cannot well be made out in paraffin sections, and in the fresh 
condition it is even more difficult to investigate the minute 
anatomy of the nervous system of the larva. From the 
examination of a very large number of sections of embryos of 
various ages, I can assert the constancy of the relations to one 
another of ganglion cells, fibrous layer and eyes, as described 
above. It is especially important to notice that the fibrous 
layer is invariably present, and can readily be found in any 
series of sections passing through a mature embryo. The 
existence of the eyes on the dorsal organ is a very strong 
a priori argument for the occurrence of nervous tissue in this 
region. 
Fig. 54 is an obliquely horizontal section, passing through 
oesophagus and intestine, brain, and vestibular invaginations. 
On the left the section cuts the lumen of the latter, but on 
the right only the cells limiting its blind end. Each of the 
two invaginations consists of more than one layer of cells, and 
