ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 257 



of a successful method of rearing the larvae throughout the entire course of their 

 development. 



In respect to both these points, we enjoyed special advantages in working at 

 Plymouth. In the first place, the development of the forms we proposed to hybridize 

 was well known in both the early and the late larval condition up to, and through, 

 metamorphosis. We were thus saved considerable preliminary embryological work. 

 In the second place, the experiments of Allen and Nelson (3) on the rearing of 

 marine plankton organisms gave us a basis on which to elaborate a method to rear 

 our larvae. 



In the following work we hope to show that we have only made use of fixed and 

 definite characters. In the pure-bred species, thioughout the course of this work 

 these characters have shown no variation whatever, although their inheritance in the 

 hybrids has not been the same throughout. They are present in one of the parent 

 forms, while absent in the other. 



We have been successful in elaborating a method of rearing our hybrid plutei in 

 large numbers through metamorphosis. We have also been able to raise a consider- 

 able number of our hybrid urchins to the fully grown condition, or to a stage when 

 the genital pores have made their appearance. 



By the discovery of the proper food of the larvae and young urchins, and the 

 optimum laboratory conditions under which to keep them, we have been able to rear 

 the hybrids throughout the entire course of their development as readily as the 

 pure-bred parent forms. 



During larval lil'e Echinoids feed on Diatoms, which can be supplied to them in the 

 laboratory from pure cultures. After metamorphosis the urchins no longer eat the 

 food on which they were reared through the larval stages. They change their food 

 as they grow. We were unable for a long time to find their proper food, but this 

 we finally succeeded in discovering, with the result that some of our hybrid urchins 

 are now over 8 cm. in diameter. We have a large number of hybrids growing in 

 the lal)oratory, and we hope to get a second generation from these next season. 

 We shall then be able to say if the inheritance of their characters follows the 

 usual Mendelian plan. 



An investigation of the cytology of our crosses has lieen kindly undertaken by 

 DoNCASTERand Gray (23), and their results form the subject of a separate publication. 

 They have established that true fertilization has taken place in all our crosses ; for 

 the material investigated by them was taken from the same batches of eggs and 

 sperm as those from wliich we successfully reared hybrids. We have raised hybrids 

 to a late stage from all the material examined by them. In this way we hoped to 

 correlate, if possible, the cytological conditions with the growtli of our hybrids. In 

 Section 12 a resume is given of their results. 



Dr. Th. Mortensen, of Copenhagen, while on a recent visit to Plymouth, kindly 

 undertook to examine and furnish us with short descriptions of our adult hybrid 



VOL. CCIV. — B. 2 L 



