The Embryology of Patella. 
23 
The projecting cells covered with fine radiating cilia, ob- 
served in the preceding stages, Pi. III, Fig. 34, 35 etc. a. c. are 
no longer distinctly visible. The tuft of long cilia in the centre 
of the apical plate retains however its former prominence. The 
apical plate has increased slightly in depth, and consequently the 
cells of which it is composed have become longer and contain 
nuclei distinguished from those of the other cells in the embryo - 
cap by their greater size and small quantity of chromatine. These 
nuclei are sitnated at the inner ends of the cells and are partly 
snrronnded with vesiculated protoplasm. The outer ends of the cells 
are filled with fine granules which extend in ward as far as the nuclei. 
Tliis granulär protoplasm contains fine lines continnous with the cilia 
upon the onter snrface and extended inward towards the nuclei. 
About the seventy-fifth hour two projections are developed 
upon the anterior snrface of the embryo-cap on each side of the. 
median line, PI. IV, Fig. 45, they are thin walled and dome-shaped 
and often filled with fine globules of a refractive snbstance. I was 
at first inclined to consider them abnormal productions, but am 
convinced that such is not the case on account of their symmetrical 
appearance and regulär occurrence at this period. By the end of 
the hundredth hour they have disappeared. 
Unfortunately, as has happened with so many other observers 
who have studied the development of the Mollusca, I am unable 
to contribute any definite resnlts as regards the development of the 
nervous System. This is due to the fact that, when the embryos had 
reached a stage old enough for the study of these changes, they had 
become so abnormally developed that any attempt to examine them 
for such a purpose would have been fruitless. The few normal 
examples of the oldest stages obtained, although sufficient to give 
the external form, were not abundant enough to afford material for 
sectioning. A few fortunate sections at this stage however will 
be of interest. PI. V, Fig. 62 is one of these sections and it will 
be observed that the apical plate is still formed of a layer of 
long cells, while on each side of it the cells have increased in 
number and formed two lateral masses. In snrface views at the 
same stage, the embryo cap-is seen to be somewhat depressed at 
the points where these lateral masses of cells should be while the 
apical plate has left its primitive position at the apical pole and 
now lies close to the velum on the ventral side of the embryo. An 
actual invagination was not seen to occur. Whether these lateral 
masses of cells increase in size and subsequently fuse to form the 
( 171 ) 
