400 



Prof. E. W. MacBride. 



[Nov. 7, 



provide the skeleton for these two arms. The lateral centre of calcification 

 is not so far developed as in the Echinocardium larva. There is, however, 

 no trace whatever of the aboral spike or of its supporting skeleton, 

 and the post-oral and postero-dorsal arms are supported by single rods and 

 not by lattice-work. In the specimen figured, however, the post-oral arm on 

 the left side is supported by two parallel rods. This is a phenomenon which 

 is by no means infrequent, and Mr. De Morgan has informed me that he 

 has often seen lattice-work in the arras of the larva of Echinus esculentus. 



The body rods in the specimen figured end in unbranched thickenings, 

 but in many specimens, especially at an earlier period of development, they 

 end in " inbent crooks," which nearly meet at the aboral pole. 



We may now examine the result of fertilising the eggs of Echinocardium 

 corclatum with the sperm of Echinus esculentus. We find that only a small 

 proportion of the eggs so fertilised develop into larvae. I estimate the 

 number at about 1 in 1,000. These hybrid larvae can be kept alive for 

 eight days ; in no case have I been able to keep them alive for longer than 

 nine days. They grow slowly, and develop only the antero-lateral and 

 post-oral arms. So far from exhibiting exclusively maternal characters 

 they show the influence of the male parent in the most unmistakable 

 manner. One of these hybrids is represented in text-fig. 3, and in it we can 

 observe several paternal characteristics. Thus the aboral spike has 

 totally failed to develop; and this cannot be explained as the effect 

 of a mere retardation of development, because, as was stated above, in the 

 normal larva of Echinocardium corclatum, before there is any trace of antero- 

 lateral arms, and when, consequently, the larva possesses only the post-oral 

 arms, and before there is any external trace of the aboral spike, there is 

 to be seen at the aboral pole of the larva a great accumulation of mesenchyme 

 cells, which in the next stage of development give rise to the skeleton of the 

 aboral spike. In the hybrid no trace whatever of such an accumulation 

 of cells at the aboral pole is to be seen at any stage of development. 

 Further, the body-rods of the hybrid show the inbending at the 

 aboral pole which we have seen to be characteristic of the larva of Echinus 

 esculentus. The lattice-work of the skeleton supporting the post-oral arms is 

 imperfectly, or not at all, developed ; this, too, is a paternal feature. The 

 maternal features which the hybrids exhibit are mainly " size " and " colour." 

 The egg of Echinocardium cordatum is less than half the size of the egg of 

 Echinus escidentus ; it is therefore to be expected that the hybrid which 

 develops from it should approximate in size more closely to the Echinocardium 

 larva than to the larva of Echinus. As a matter of fact, it is considerably 

 smaller than either. The pigment spots of the Echinocardium larva are of 



