IGi EDOUARD VAN BENEDEN. 



animals.^ Nothing, however, is easier than to distinguish 

 the ovaries from the testicles. It is true that the sexual 

 organs have the same form, the same position, and the same 

 volume in the two sexes ; when they have reached their 

 complete development the five pairs of sexual glands extend 

 along the entire length of the arms, and they cause a marked 

 elevation of the skin of the side of the back, so that one can 

 recognise, even from the external appearance, the individuals 

 in which the sexual products have attained maturity. The 

 most superficial microscopic examination suifices to distin- 

 guish the contents of the testicle from those of the ovaries, 

 and one soon learns to recognise the sex, even with the 

 naked eye ; the ovaries have a yellowish or very pale 

 brownish tint ; the testicles are of a pure milk-white colour. 

 Moreover, the lobules of the ovarian clusters are more 

 rounded and shorter, whilst those of the male sexual gland 

 are elongated and rather of tubular shape. 



I will first describe the ovarian ovum, such- as it appears 

 when, already free in the cavity of the ovary, it has attained 

 the dimensions of the ripe ovum, but while its germinal vesicle 

 is still lodged in the centre of the yolk. These ova may have 

 either an ellipsoid form, or else they may be pyriform. Their 

 dimensions vary between '16 by '13 and '19 by '17 milli- 

 metres. They are composed of a thick and entirely homo- 

 geneous envelope, of a finely granular yolk, and of a germinal 

 vesicle which is situated in the neighbourhood of the centre 

 of the yolk. 



Membrane. — It is still a question whether there exists 

 around the ovum of the Asteridea a single one or two mem- 

 branes; nor is anything more certain known as to the nature 

 and signification of these envelopes. If fresh ova which have 

 reached the dimensions of the ripe ovum, and which still have 

 the germinal vesicle central, are examined, we can distin- 

 guish a clear zone around the yolk which has a thickness of 

 from '003 to '004 millimetres. This is quite clear, trans- 

 parent, and homogeneous. It is limited where in contact 

 with the yolk by a sharply defined contour ; on the outer 

 aspect, on the contrary, its contour is pale and so slightly 

 marked, that it requires great attention to discern it. The 

 index of refraction of the substance which constitutes this 

 membrane must be very similar to that of water. This 

 substance is very soft ; it appears to be a gelatinous, muci- 

 laginous, or albuminoid body ; whence the names of 

 " Gallerthiille, Eiweisschicht, and mucilaginous layer" which 



' Tiedemann, ' Anatomic der Rohren-Holothurie, des Pomerauzfarb. Sees- 

 terns,' &c., 1816, p. 42. 



