STUDIES ON THE DEVELOPMENT OK ECHINOIDEA. 305 
developed all eight ' arms. These differences concern the 
arrangement of the ciliated epaulettes, and the appearance of 
a green pigment in the larvae of Echinus miliar i s; they were 
described by me in 1899 (15). If anyone unacquainted with 
the details of the normal development of Echinoderm larvae 
were to read thi'ough the papers of all the workers on hybridi- 
sation he would never suspect that the Echinoplutens 
larva ever developed more than four arms; and in this cir- 
cumstance alone a strong instance is afforded of the necessity 
of knowing the ordinary development of a species before we 
make it the subject of “ experimental embryology.” In his 
first paper Boveri actually described the larva of Sphfer- 
echinus as possessing only' two arms ! Whilst I was seeking 
an opportunity to begin work the questions of the result of 
hybridising Echinus esculentus and Echinus miliar is 
was taken up by Shearer, De Morgan and Fuchs, who pub- 
lished a preliminary note of the results of their work (19). 
In this note they maintain that the hydirid between these two 
species is of a purely maternal chai'acter with respect to 
whatever character be selected for examination ; and they 
maintain that this maternal character is not altered by 
changing the acidity or alkalinity of the sea- water employed. 
This certainly is a remarkable result to obtain by hybridising 
two speciesof the samegenus,and experiments which have been 
made at the Imperial College, the results of which will shortly' 
be published, do not bear out the view of these authors. 
The second case forms the subject of the present paper. 
Vernon (25) had already' recorded the results of crossing the 
eggs of two speciesof Echinocardium with the sperm of 
Arbacia, E chinus, Sphaorechinus and Strongylocen- 
trotus,and he found that the hybrid larvm were all of a purely 
maternal type. When the sperm of Echinocardium was 
used to fertilise the eggs of the other genera no result was 
obtained except in one instance with the eggs of Echinus. In 
this case one third of the eg-gs developed and produced larva; of 
a ])urely maternal type. The great distinguishing feature of the 
larvae of Echinocardium, which it shares with the larvae 
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