EMBRYOLOGY. 719 



base of the arms e and e", in continuation of the chord of cilia extending 

 along these arms. 



Figs. 52, 53, represent embryos twenty-three days old ; during the next four 

 days no change of any importance could be perceived ; the tentacular loop 

 remained the same, the arms alone increasing in size, and a few dark pigment 

 spots appeared in the arms. Unfortunately, at the end of these four weeks 

 the young embryos all died. I have only once succeeded in keeping 

 them such a long time, and that was during the coldest winter weather. In 

 the attempts made in the spring, whenever a warm day came, it was sure to 

 kill everything ; while in the summer, though the facilities I had were in- 

 finitely greater, I never could keep these embryos alive more than three or 

 four days. The sea-urchins spawn during the whole year. Successful arti- 

 ficial fecundations have been made in December, January, and during even- 

 month from that time till the middle of October. 



The remaining observations on the development of this species were all 

 made from specimens caught swimming on the surface of the water ; this 

 applies to the fully-formed sea-urchins as well as the pluteus, only some of the 

 most advanced specimens (PI. X.f. 2 ; PL IX.) were found thrown upon the 

 beach after a storm, attached to Laminaria. The specimens obtained in this 

 way, the various stages of which were traced until there coukl be no doubt 

 to what species they belong, connect so nearly with specimens obtained from 

 artificial fecundation as to leave but few gaps to fill in the plutean condition 

 to give us all their transformations. The sea-urchins raised from embryos 

 caught swimming freely about were kept in confinement until they had at- 

 tained the size of some of the more advanced nomadic sea-urchins (PL X. 

 f. 2). This can leave no doubt to which of our two species of Echini these 

 embryos should be referred. 



The next oldest pluteus, which is Fig. 54, shows that since the last stage 

 represented the principal changes have taken place in the oral part of the 

 pluteus ; the arms e", e", especially, have greatly increased in length, the 

 outline of the anal extremity is somewhat rounded, the rod which runs 

 along its edges is made up of short, stout pieces with strong pointed pi-ojec- 

 tions, and the rods of the arms are composed of three rods connected to- 

 gether by transverse spokes : it requires close examination to distinguish 

 this. On the aboral side two very prominent spurs project over the stomach, 

 somewhat below the point of junction of the rods of the arms e', e", e" . Ad- 

 ditional tentacular loops have been formed ; we can distinctly trace three on 



