474 
B. W. MACBIUDE. 
Formation of antero-lateral arms and of 
aboral process ..... 3 days 
First trace of postero-dorsal arms . . 4 
Postero-dorsal arms fully developed ; first 
trace of Ecliinus rudiment and of 
prae-oral arms . . . . , 9 „ 
Hydrocoele with incipient lobes; first 
trace of postero-lateral arms . 10-12 ,, 
Formation of antero-dorsal arms ; tube- 
feet protruding with amniotic cavity 14-16 
Formation of adult spines; absorption of 
aboral process , . . . 18-22 „ 
Metamorphosis complete . . . 24-30 
The egg is not a sphere bat an ellipsoid^ and its shape 
corresponds to that of the blastula which is developed frOTu 
it (fig. 1). The egg of Echinus is spherical and gives rise 
to a spherical blastula; it follows that the shape of the 
blastula, since it is conditioned by the shape of the egg, is 
always a maternal character. At the aboral pole of the 
blastula of Echi n ocar di u m there is a patch of thickened 
epithelium consisting of long, narrow, filamentous cells bearing- 
long cilia (ap., fig. 1) ; this patch, like the apical plate of the 
annelid larva, must be regarded as a sensory organ, the 
purpose of which is to direct the tiny larva in its course 
through the water. No such organ can be detected in the 
blastula of Echinus which progresses by a rolling motion, 
but in the gastrula of Echinus, in which the rolling motion 
gives place to motion along a definite line, a very similar 
organ can be detected. 
As in other Echinoid larvae mesenchyme cells (p. mes., fig. 1) 
are given off from the pole of the blastula opposite the one at 
which the sensory organ is situated, before any sign of the 
invagination which is to form the archenteron can be detected. 
The main purpose of this mesenchyme — which is termed the 
pri mary m esenchy me, is to form the basis in which the rods 
of the larval skeleton are secreted. By the close of the second 
day not only is the archenteron completely formed, but the 
