122 BULLETIN OF THE 



sperm. Many eggs can be washed out of the undissected sea-urchin 

 with a pipette introduced through the aboral region. The floating eggs 

 and the milky sperm are mixed by simply pouring the water from one 

 jar to the other. A better method of artificial fecundation is to collect 

 a watch glass crystal full of eggs, leaving enough water for them to 

 float, and then to drop a few drops of water charged with sperm among 

 them. The contents are then gently stirred, and after a short time 

 evidences of the success of the process may be looked for. I have 

 found that chopping up the two glands together, although in some cases 

 to be recommeuded, in most instances, and especially in the case of 

 Echinarachnius, the egg of which is delicate, leaves so much decaying 

 matter that the ova are killed. It is well not to put too much water 

 with the ova, as repeated dilution renders the collecting of the ova for 

 study difficult. I took no precautions about the temperature of the 

 water, and did not find it necessary to change the water until after 

 segmentation was finished.* Artificial fecundation was accomplished 

 from the middle of July to the end of August. 



Each ovum, PI. II. fig. 1, is visible to the naked eye. It is sur- 

 rounded by a viscous 1 layer in which are beautiful, spherical or some- 

 times angular, red pigment spots, pig, which are supposed to correspond 

 to the "clouded areas," described by A. Agassiz t in the star-fish egg. 

 The viscous layer of the egg of E. esculentus is described by Derbes. 

 A. Agassiz describes a ** thick homogeneous structureless shell " in 

 Strongylocentrotus. The pigment spots are conspicuous on the outei 

 surface of the viscid capsule of the egg of Echinarachnius. After fer 

 tilization the ova sometimes sink and sometimes remain floating. Theii 

 specific gravity is about that of the water. 



The diameter of the yolk, t;<, is .13 mm. The diameter of the viscid 

 covering is from .22 to .25 mm. The yolk is yellow ; the envelop trans- 

 parent. The yolk was not observed in the free egg to fill its capsule in 

 any stage or segmentation. 



A nucleus and nucleolus were observed in ovarian eggs. These 

 structures were difficult -to see in free eggs. 



The spermatozoa immediately after the mingling of the two sexual 



* Selenka and others have already pointed out refined ways of fertilizing sea- 

 urchin eggs. See Selenka, " Keimblatter und Organenlage der Echinodermen." 

 Zeit.f. Wiss. Zool, XXXIH. p. 40. 



t Similar pigment spots are found according to Nachtrieb in the egg of Mellita. 

 These spots on the ovum of Echinarachnius were first described in my paper on the 

 development of Arbacia. Mem. Peahody Acad., I. 6. 



