ECHINOCYAMUS PUSILLUS. 7 



II. 



The OTiim, its maturation and impregnation. 



The immature but full-sized egg, taken from the ovarial tubes, has 

 a diameter of 0,12 mm. including its enveloping mucilaginous coat. It 

 consists of a finel}^ granulated, light yellowish yolk with a more trans- 

 parent and homogeneous peripheral layer, and contains a rather large 

 eccentric germinal vesicle with a diameter of 0,06 mm., a well deve- 

 loped protoplasmic nuclear reticulum and a round eccentric germinal 

 spot measuring from 0,oo8 mm. to 0,oi2 mm. in diameter. Besides, the 

 vitellus is enveloped in a glassy mucilaginous coat with slightly uneven 

 contours and a thickness of from 0,oo4 mm. to 0,oo8 mm. Though I exa- 

 mined this coat with the highest power, I never succeeded in distin- 

 guishing any distinct structure in it, and consequently there can be no 

 true zona radiata in Echinocyamus. On the contrary, the envelope often 

 appeared finely striated in a concentric manner. 



These unripe eggs are closely packed together in the genital 

 tubes and only separated from each other by the glassy envelope, and 

 from a section of such a tube we learn that they have a more or less 

 irregular rounded or oval form. 



The ripe ovarial egg is slightly larger, about 0,i6 mm. in diameter. 

 Its enveloping coat swells considerably, acquiring a thickness of up to 

 0,032 mm. or more, and assumes such a degree of transparency that it 

 becomes almost invisible, being traceable only by the action of some 

 colouring matter. The »beautiful, spherical or sometimes angular, red 

 pigmented spots», which Fewkes ') has found in the »viscous layer» of 

 the eggs of Echinarachnius and Nachtrieb ^) also in the ova of Mellita, 

 do not exist in the eggs of Echinocyamus. The yolk of the ripe ovum 

 is rather transparent and finely granulated with uniformly distributed 

 deutoplasm, and its very thin peripheral layer of clearer protoplasm is 

 surrounded by a delicate membrane with apparently double contour. I 



1) Preliminary Observations on the Development of Ophiopholis and Echina- 

 rachnius. — Bull. Mus. Comp. Zoology at Harvard College. Vol. XII. N:o 4. Cam- 

 bridge 1886. p. 122. 



2) Preliminary Notes on the Echiuoderms of Beaufort. — Johns Hopkins 

 University Circulars, Vol. IV. N:o .38. Baltimore. March 18S5. p 67. 



