120 THE ANATOMY OF VEETEBEATED ANIMALS. 



part of the cephalic region (Fig. 29, a), ought to be con- 

 sidered as an olfactory organ. 



The myelon traverses the whole length of the spinal 

 canal, and ends anteriorly without enlarging into a brain. 

 From its rounded termination nerves are given off to the 

 oral region, and to the rudimentary eye or eyes (Fig. 29, 

 b, c). 



According to M. Kowalewsky,* who has recently studied 

 the development of AnqiMoxus, the vitellus undergoes com- 

 plete segmentation, and is converted into a hollow sphere, the 

 walls of which are formed of a single layer of nucleated cells. 

 The wall of the one moiety of the sphere is next pushed 

 in, as it were, until it comes into contact with the other, 

 thus reducing the primitive cavity to nothing, but giving 

 rise to a secondary cavity, suiToimded by a double mem- 

 brane. The operation is, in substance, just the same as that 

 by which a double nightcap is made fit to receive the head. 

 The blastoderm now acquires cilia, and becomes nearly 

 spherical again, the opening into the secondary cavity being 

 reduced to a small aperture at one pole, which eventually 

 becomes the anus. M. Kowalewsky points out the resem- 

 blance, amoimting almost to identity, of the embryo at this 

 stage with that of many Invertebrata. 



One face of the spheroidal blastoderm becomes flattened, 

 and gives rise to lamincB dorsales, which unite in the charac- 

 teristically vertebrate fashion ; and the notochord appears 

 between and below them, and very early extends forwards, 

 beyond the termination of the neural canal. The neural 

 canal remains in communication with the exterior, for a 

 long time, by a minute pore at its anterior extremity. The 

 mouth arises as a circular aperture, developed upon the 

 right side of the anterior end of the body, by the coalescence 

 of the two layers of the blastoderm, and the subsequent 

 perforation of the disc formed by this coalescence. The 

 branchial apertures arise by a similar process, which takes 

 place behind the mouth ; and they are, at first, completely 



* ' Meraoires de TAcademie Imperiale des Sciences de St. Peters- 

 bourg,' ]S67. 



