310 
PEOF. E. VV. MACBRIDE. 
forward the view that tlie chemical action of the sperma- 
tozoon in provoking development consists of two parts ; 
first a process of cytolysis is set np in virtue of which 
a vitelline membrane is formed, but this process would lead 
to the destruction of the egg if it were not checked ; accord- 
ingly a second chemical action sets in in virtue of which 
the first action is arrested. In artificial parthenog'enesis 
the checking of the process of cytolysis is effected by the 
hypertonic sea-water. 
I Avas curious to see what actually had happened to the 
Echinus egg when treated with the sperm of Echino- 
cardium. Microscopical examination revealed the fact 
that the eggs had formed fertilisation membranes, showing 
that the spermatozoa had entered them, but that they had 
then undergone cytolysis. One such egg is shown in fig. 1, 
PI. 14. The spermatozoa of Echiuocardium were therefore 
able to produce a cytolytic action, but incotnpetent to check 
it when it went too far. 
Here, then, the cause of the sterility of the cross has been 
unmasked. 
Turning now to the experiments in which Echinocardium 
eggswere fei'tilised with Echinus sperm Ishall divide what 
1 have to say into three parts. First I shall describe carefully 
the normal development of Echinus esciilentns up till the 
sixth day; then I shall describe that of Echinocardium 
cordatum up till the fourth day; and then I shall describe 
the development of the hybrids, which, although they lived 
for eight days, did not attain a greater degree of development 
than that reached by the normal larvae in about five days. 
(1) '1'hk Development oe Echinus esculentds. 
On the morning following the fertilisation of the eggs, 
which had been accomplished in the previous afternoon, clear 
spherical blastulae (PI. 14, fig. 2) were seen swimming at the 
surface of the Avater. The vegetative pole Avas already marked 
by the appearance of the first mesenchyme cells, but they had 
not yet migrated into the blastocele. Soon the blastula 
