GERMINAL VESICLE AND FIRST EMBRYONIC NUCLEUS. 1G5 



have been given to it. More recently Hoffmann^ has called 

 it the " vitelline membrane.''^ To see well the characters of 

 this membrane, examination in water must be avoided. The 

 moment it comes in contact with water it swells con- 

 siderably ; one may study it conveniently by placing the 

 eggs directly in 1 per cent, solution of osmic acid, or in a 

 concentrated solution of picric acid. It is not coloured red 

 by the colouring matter of carmine fluids; it is partially 

 dissolved or contracts strongly by a prolonged stay in 

 absolute alcohol. After remaining for a certain lengtli of 

 time in absolute alcohol it is only a very thin membrane 

 which immediately envelops the vitellus, and which at 

 certain points is in immediate contact with it, whilst at 

 other points it is separated from it, thus forming more or less 

 regular undulations. If one examines the transparent layer 

 in ova which have not attained their complete development, 

 it exhibits a radiated striation which is due to the presence of 

 pores in the form of canaliculi of extreme tenuity; it has 

 been long known that the ova of Holothurians present the 

 same character. 



I have never observed in the ova of the sea starfish the 

 canal which J. Miiller has discovered in several Holothuria, 

 and which has been thought to fulfil the function of a micro- 

 pyle. However, in the pearshaped ova the transparent 

 layer is oftentimes a little thinner at the tail end of the pear. 



Derbes has found in the ovum of the Echinida, indepen- 

 dent of the mucilaginous zone, which I have just described, 

 a thin membrane which is immediately applied to the sur- 

 face of the vitellus, and which he calls a vitelline membrane. 

 Several authors have affirmed since Derbes, the existence of 

 this second membrane, not only in the Echinida, but also in 

 the Holothurida and Asterida. I have not been able to convince 

 myself of the existence of this membrane in the %^^ of the 

 Aster acantliion ruhens. 



What is the signification of the transparent layer which 

 exists around the ovum in all the Echinodermata ? What 

 name must be given to it ? It is not possible to answer these 

 questions in the present state of our knowledge on the forma- 

 tion of the e^^ of these animals. I have reasons for believ- 

 ing that the membrane is not produced by the egg itself, and 

 that its mode of formation is the same as that of the zona 

 pellucida of mammals. If my opinion is correct, there 

 would be grounds for designating it by the name of Chorion. 

 As its microscopical characters have some resemblance to 



' C. K. Hoffman, " Sur 1' Anatomic des Asterides," ' Extrait des Ar- 

 chives Neeriandaises,' vol. ix. 



