xvi ECHINODERMATA 505 



On the east coast of North America we find the species 

 Strongylocentrotus droebachiensis under similar conditions in count- 

 less thousands, and as the larva of Strongylocentrotus cannot be 

 distinguished from the larva of Echinus by any certain mark, all 

 that will be said about Echinus esculentus may be taken as apply- 

 ing to Strongylocentrotus. On the Pacific coast Strongylocentrutas 

 droebachiensis is replaced by the larger species S. purpiiratus and 

 S.franciscanus, the eggs of which are equally suitable for rearing. 



METHOD OF OBTAINING NORMAL EMBRYOS 



The urchins are most easily opened by inserting one blade of 

 a pair of scissors through the margin of the peristome, and then 

 cutting along a meridian of the shell till the equator is reached ; 

 the cut is then carried completely round the equator and so the 

 shell is separated into an upper and a lower half. In the upper 

 half will be found the genital organs. 



The ripeness of the testes can be ascertained by squeezing out 

 a drop of the spermatic fluid into a drop of sea-water, when a 

 glance at the mixture through the low power of a microscope will 

 show whether the spermatozoa are active or not. 



The ripeness of the eggs can be gauged by two criteria : (1) 

 whether they break loose from the ovary at the slightest touch, so 

 that the ovary appears to dissolve into dust ; (2) whether the large 

 germinal vesicle, so characteristic of the unripe egg, has dissolved. 

 Bolton cloth of a mesh of -i- mm. is a useful adjunct. If a piece of 

 ovary be wrapped in a piece of this cloth and shaken into clean 

 sea-water, the eggs, which easily detach themselves, will escape 

 through the meshes of the cloth, and the younger undeveloped ova 

 and follicle cells, which adhere to one another, will remain behind. 



A better method of obtaining ripe eggs, however, is to invert 

 the upper half of the bisected urchin in a small glass dish containing 

 some sea-water. The ripe eggs will then be discharged practically 

 without admixture of unripe ones, an admixture which it is impossible 

 to avoid when pieces of the ovary are shaken out into sea-water. 



If the eggs are perfectly ripe they will be free from all mem- 

 branes ; but they show a thick glassy chorion, just like that which 

 we have described in the eggs of Asterias glacialis, which prevents 

 the easy access of spermatozoa. If, however, these eggs are allowed 

 to lie for two hours in clean sea-water before being fertilized, the 

 chorion will completely disappear. Fertilization can, however, take 

 place before the chorion disappears. 



In fertilizing, care must be taken that the eggs are spread out 

 over the bottom of the vessel in a single layer, and that a very 

 small quantity of spermatozoa is added. In practice, an emulsion 

 of spermatozoa is prepared by shaking up a drop or two of thick 

 spermatic fluid with sea-water in a specimen tube four or five 

 inches long. If the eggs are contained in a glass evaporating dish 



