816 
PROF. F. \V. MACBRIDE. 
the rudiment of its skeleton (p.d.). The ccelomic sacs, which 
in the previous stage lay at the sides of the oesophagus, now 
begin to grow back along the sides of the stomach and on 
each side a madreporic pore is formed. Whether this 
duplicity of the madreporic pore is constant or not I have not 
yet been able to determine ; it is, at any rate, very frequent. 
A larva in this stage of development is represented in fig. 11. 
Further than this stage it is not my intention to pursue the 
history of the development. The larva of Echinocardium has 
in four days reached the same stage of development as that 
attained by the larva of Echinus esculentus in six days. 
Let us now review the differences between the two types of 
larvae. Leaving out the question of the duplicity of the 
madreporic pore, which may turn out to be a most important 
character but which cannot be used at present — we find that 
the larva of Echinocardium differs from that of Echinus — 
(1) In possessing an aboral spike supported by a com- 
plicated skeleton, whilst at the aboral pole of the Echinus 
nothing of this kind is to be seen. 
(2) In possessing ‘latticed” bars instead of single rods 
as supports for the post-oral arms. (Exceptionally in 
Echinus a second rod can appear in the post-oral arm. A 
trace of such rod is represented in fig. 4. I have never seen 
cross-connections between the two, but I am informed that 
these also can appear as a rare exception.) 
(3) In possessing well-developed recurrent rods instead 
of only vestiges of such rods. 
(4) In possessing brownish-red instead of orange-red 
pigment. 
(5) In its more rapid development. 
We shall now see how far these characters are represented 
in the hybrids. 
Development of Eggs op Echinocakdium Cordatum Fertilised 
WITH THE Sperm of Echinus Esculentus. 
The earlier course of the development of Echino- 
cardium eggs fertilised with the sperm of Echinus 
