424 
W. A. HA SWELL. 
are 0’0075 mm. in diameter, with correspondingly small 
nuclei. No definite arrangement of these cells is recognis- 
able until a few of the larger cells become grouped together 
(PI. 24, fig. 5) in such a way as to bound a small rounded 
cavity. This elongates and widens (PI. 24, fig. 6), the bound- 
ing cells meantime increasing in number. Eventually (PI. 24, 
figs. 7, 8, 9, and 10) the space dilates very greatly, the cells 
which form its walls becoming- correspondingly extended, 
and uniting together to form a comparatively thin membrane 
with flattened nuclei. As it enlarges, this space becomes 
approximated towards the surface of the egg, coming to be 
separated from the shell only by a thin layer of yolk. In 
apposition with the deeper side of the space lies the main 
mass of the blastoderm, which is rapidly increasing in extent, 
the increase being mainly due to the multiplication of the 
middle-sized cells. 
The space above referred to does not correspond, so far as 
I have been able to ascertain, to anything that has been 
found to occur in any other group of animals. Since it plays 
an important part in development, it is necessary to have a 
name for it, and I propose the term endocoele as one not 
involving any dubious homologies.' 
The rudiment of the brain (PI. 24, figs. 8, 9, and 10, hr.) 
makes its appearance as the endocoele approaches its maxi- 
mum size. It appears first as a bilobed, dense aggregation 
of cells on the deeper dorsal side of the endocoele, a little 
distance from the lining- membrane. In the middle of this 
appears a transversely elongated space filled with finely 
fibrillated matter — “ Punktsubstanz.” Nerve-fibres (or nerve- 
tubes) are only developed in the latest embi-yonal stages. 
From the central mass a pair of processes — the foundations 
of the peripheral nervous system — are given off laterally. 
About the same time as the beginnings of the nervous 
system, appears the first rudiment of the excretory system of 
vessels. This takes the form of several speci:illy modified 
* Tlie same cavity with the same relations occurs in Craspedella 
as well as Temnocephala. 
