244 



Studies in Heredity. 



to me that a critic might possibly advance the theory that the eggs which 

 I believed to have undergone cytolysis, in consequence of the entry of foreign 

 sperm, really died in the normal course of affairs through the inability of 

 the foreign sperm to enter them and the consequent absence of fertilisation. 

 In the case of Echinus eggs the reply might be made that these even when 



Calc, 



Int. 



Fig. 4. — Stage equivalent to the "prism-larva" in a hybrid produced by fertilising the 

 egg of Echinus esculentus with the sperm of Echinocardium cordatum. Gale, incipient 

 calcifications, the rudiments of the larval skeleton ; Int., intestine ; St., stomach ; 

 Stom., stomod^um (ectodermal oesophagus). 



undergoing cytolysis show clearly and unmistakably the fertilisation mem- 

 brane, which may be accepted as evidence that a spermatozoon has entered 

 the egg, but the egg of Echinocardium (fig. 1) does not show the fertilisation 

 membrane clearly ; if present it lies close to the surface of the egg. In order 

 therefore to place the matter beyond doubt the following experiment was 

 made : — 



Eggs were shaken out from a ripe Echinocardium into sterilised sea-water. 

 The culture thus obtained was divided into two portions : one portion (A) 

 was treated with the sperm of Echinus esculentus, and one portion (B) was 

 left unfertilised. On the following morning, in (A) all the eggs were dead 

 and had undergone cytolysis, but all the eggs in (B) were as clear and 

 transparent as when they had left the parent. (B) was now divided in 

 three portions, which we shall designate (a), (b), and (c) ; (a) was fertilised 

 with Echinocardium sperm, (b) was fertilised with Echinus sperm, and 

 (c) was left unfertilised. On the following morning, (a) had given rise to 

 numerous larvae, showing that the eggs of which it was composed were still 



