Characteristics of the Verteb'rata. xxxix 



Cyclostomi and Amphibia ?) is formed by the junction to the third 

 layer, which has the shape of a groove open towards the yolk 

 cavity, of an outer fibrous covering, due to the splitting into two 

 portions of the ventral part of the middle layer ; and by the sub- 

 sequent conversion of the demi-canal, thus formed, into a tube at 

 its two ends. The space contained between the two layers into 

 which the downward prolongation of the middle layer divides 

 itself, corresponds with the future pleuro-peritoneal cavity of the 

 adult Vertebrate ; and the orifice and canal of communication 

 betwen the yolk cavity and the tube which the demi-canal is thus 

 converted into, correspond with the more or less transitory om- 

 phalo-mesenteric duet, which connects the vertebrate intestine with 

 the umbilicus. In no Invertebrate animals are the walls of the 

 perivisceral cavity thus constituted ; and in none does the intestine 

 ever possess any umbilicus. In the Amphioxus, however, (as also 

 in Cyclostomi and Amphibia ?) the intestinal tract is said to be 

 formed, as it is in many Invertebrata, by a process of invagination 

 commencing at the future anus -, and the larvae of certain Asci- 

 dians have been stated to have their nerve-centre developed similarly 

 to the tubular cerebro-spinal centres of Vertebrata, and to possess 

 within the locomotor caudal appendage, Avith which they are fur- 

 nished in their larval condition, a structure closely similar to the 

 cJiorda dorsalis of Vertebrata. 



In all Vertebrata, with the exceptions just mentioned, a larger 

 or smaller ' umbilical vesicle^ is formed by the separation of a distal 

 or extra-abdominal portion of the yolk-sac from an intra-abdominal 

 moiety, at the point where the ventral laminae close upon it in the 

 medio-ventral line, and form the ^ umbilicus.^ The ' umbilical ve- 

 sicle'' is usually cast off when the embryo is set free from the egg ; 

 the part of the yolk sac which is intercepted within the abdominal 

 cavity, frequently persists for a considerable period after birth as 

 the ' omphalo-mesenteric duct.' 



Divisions, AUantoidea and Anallantoidea. 



Vertebrata are divided into Amniota and Anamniota, accordingly 

 as the dermal and cuticular elements of the ventral laminae are in 



