242 Mr. E. W. MacBride. 



repeat them here. Unfortunately I could not keep any of them alive for a 

 longer period than five days, and therefore I did not obtain one of the features 

 (viz., the inbending of the aboral ends of the body rods) which only appeared 

 in hybrids which in 1911 had lived longer than five days. 



Few as were the hybrids obtained by fertilising the eggs of Echinocardium 

 with the sperm of Echinus, still fewer were the hybrids resulting from the 

 fertilisation of the eggs of Echinus with the sperm of Echinocardium. In 

 the vast majority of cases the eggs broke up with the formation of globules 

 as I described in 1911. In the case of one urchin I obtained larvae which 

 were purely maternal in character, i.e. which resembled exactly the normal 

 larvae of Echinus esculentus. This result therefore tallied with that which 

 Fuchs had obtained in 1912, indeed the larvae which I obtained lived longer 

 and developed further than those described by Fuchs. In all other cases 

 very peculiar hybrids were produced. The egg segmented so as to form a 

 regular blastula, but when the primary mesenchyme cells were budded off 

 into the blastocoele (fig. 2) the divergence between the hybrid and normal 



Fig. 2. — Blastula produced by fertilising the egg of Echinus esculentus with the sperm of 



Echinocardium cor datum . 



larvae became clearly marked. In the normal blastula only about fifty 

 primary mesenchyme cells are produced, all of about the same size, and 

 these are arranged in a ring, but in the hybrid not only are the mesenchyme 

 cells of many different sizes but some of them show the beginnings of the 

 process of cytolysis. The conclusion is irresistibly suggested to the mind 

 that this over-production of mesenchyme is really a manifestation of the 



