228 Mr. H. M. Vernon. The Relations between the 



"The Relations between the Hybrid and Parent Forms of 

 Echinoid Larvae." By H. M. VERNON, M.A., M.B. Com- 

 municated by Professor RAY LANKESTER, F.R.S. Received 

 March 29, Read May 5, 1898. 



(From the Zoological Station, Naples.) 

 (Abstract.) 



The object of this research was to determine systematically during 

 a period of several months' duration, the exact relationship of structure 

 and size existing between certain hybrid and parent Echinoid larval 

 forms. Eight different species of Echinoids were worked with, but 

 the larger number of observations were confined to three of them, 

 viz., Strongylocentrotus lividus, SpJicerechinus granularis, and Echinus 

 microtuberculatus. The method of procedure was similar to that 

 described in a former paper.* It consisted in shaking pieces of the 

 ovaries and testes of the various Echinoids in small jars of water, 

 and then mixing portions of the contents. In the cross-fertilisations, 

 precautions were of course taken to prevent any accidental direct 

 fertilisation of the ova. After standing an hour, the now fertilised ova 

 were transferred to large jars of water, holding, as a rule, 2 to 3^ 

 litres. Here they were allowed to develop for eight days, when the 

 plutei formed were killed, preserved, and examined under the micro- 

 scope. The structure of the hybrids, in relation to that of the parent 

 forms, was studied ; and all the larvae, both pure and hybrid, were 

 measured by means of a micrometer eye-piece in respect of their 

 body length and anal arm length. Fifty larvae were, as a rule, 

 measured in each case. In addition, the ova were examined under 

 the microscope twenty-four hours after fertilisation, and the numbers 

 of blastulae and unfertilised ova in a given volume of water counted. 

 Again, after eight days, the number of plutei surviving was similarly 

 estimated. 



Upon the cross SpJicerechinus ? Stronyylocentrofus <?, twenty-two 

 experiments were made. As a rule only about 10 per cent, of the ova 

 were fertilised, and only 1 per cent, of them reached the eight days 

 pluteus stage. The hybrids were most easily obtained in the summer 

 months, few or none of the ova being cross-fertilised in the winter, 

 unless the aids to fertilisation made use of by O. and R. Hertwig,t 

 and by Born,J were adopted. Thus the former observers showed 

 that if the ova were shaken in water and kept some hours, so that 

 their vitality became diminished, they underwent cross-fertilisation 



* ' Phil. Trans.,' B (1895), p. 577. 



f ' Jenaische Zeitschrift f. Medicin,' vol. 19, p. 121 (1886). 



J ' Pfliiger's Archiv,' vol. 32, p. 453. 



