166 EDOUARD VAN BENEDEN. 



those of the zona pellucida of Mammalia, we may, at least 

 provisionally, call it by that name which has the advantage 

 of recalling its physical characters without in any way 

 prejudging its equivalence from a morphological point of 

 view. 



Vitellus. — The yolk is formed of a clear and transparent 

 fundamental substance (protoplasm) and of feebly refracting 

 vitelline granules held in suspension in the protoplasm. 

 These granules are formed of a substance, the refractive 

 index of Avhich is but little greater than that of the vitelline 

 protoplasm. Hence it results that the transparence of the 

 ovum is scarcely altered by their presence. The absence from 

 the yolk of all vesicular or globular elements and of all highly 

 refractive substances, causes the body of the ovum to be far 

 from the appearance of an emulsion. It is a clear, transparent 

 and finely granular mass. This circumstance renders the 

 eggs eminently favorable to the study of the modifications 

 which the germinal vesicle undergoes in the c^^ which has 

 reached maturity. 



It is possible to distinguish \\\ the yolk of the ova of the 

 sea starfish two layers, or if the expression be preferred, two 

 substances ; a cortical layer, the thickness of which is nearly 

 equal to one third of the radius of the yolk, and a medullary 

 mass. The cortical layer is clearer and less granular than 

 the medullary mass ; it presents, moreover, a slight radiated 

 striation which appears to me to be wanting in the medullary 

 mass. The limit between the two constituent parts of the 

 vitellus is not marked by a very clear line, the cortical sub- 

 stance of the ovum passes insensibly into the medullary sub- 

 stance. Yet the zone of transition is very narrow. This 

 distinction between the two constituent substances of the 

 ovum of the Asterida has escaped the observation of all those 

 who have studied the sexual products of these Echinoderms. 

 It has not been mentioned up to the present time in any 

 animal of this division, and I am surprised that Hertwig, 

 who has studied the ova of the Toxopneustes with so much 

 care, should not have observed it. 



Germinal vesicle. — The germinal vesicle is perfectly spheri- 

 cal ; it is lodged in the centre of the medullary mass of the 

 eg^. It is marked out by a very sharply defined and deep 

 line. It encloses a transparent and perfectly homogeneous 

 liquid. If we examine the germinal vesicle whilst it is 

 in the yolk, we perceive in this liquid a voluminous 

 and very conspicuous germinal spot, and around it a cer- 

 tain number of much smaller globules which are pseudo- 

 uucleoli. The germinal spot, or spot of Wagner, is of large 



