THE PHAEYNGOBRANCHII. 107 



the development of Amphioxus, the vitellus undergoes com- 

 plete segmentation, and is converted into a hollovv 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 re- 

 ducing the primitive cavity to nothing, but giving rise to a 

 secondary cavity, surrounded by a double membrane. The 

 operation is, in substance, just the same as that by which a 

 double nightcap is made fit to receive the head. The blasto- 

 derm 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 resemblance, amounting 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 forward be- 

 yond 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 disk 

 formed by this coalescence. The branchial apertures arise by a 

 similar process which takes place behind the mouth ; and they 

 are, at first, completely exposed on the surface of the body. But, 

 before long, a longitudinal fold is developed upon each side, and 

 grows over the branchial apertures. The two folds eventually 

 coalesce on the ventral side, leaving only the abdominal pore 

 open. One cannot but be struck with the resemblance of these 

 folds to the processes of integument which grow over the bran- 

 chiae of the amphibian larva ; and, in like manner, enclose a cavi- 

 ty which communicates with the exterior only by a single pore. 



In a great many of the characters which have been enu- 

 merated — as, for example, in the entire absence of a distinct 

 skull and brain, of auditory organs, of kidneys, of a cham- 

 bered heart ; in the presence of a saccular liver, of ciliated 

 branchiae and alimentary canal ; and in the extension of the 

 notochord forward to the anterior end of the body — ■Amphi' 

 (xntx differs from every other vertebrated animal. Hence 

 Prof. Haeokel has proposed to divide the Vertebrata into 

 tvro primary groups — the Leptocardia, containing Amphi' 



