46 Prof. M'Intosh's Notes from the 



better defined than the pseudo-eliordoid tissue whieh occupies 

 the convex region ventrally (PI. VI. fig. 33). The ehordoid 

 axis soon increases in bulk, and fills the stalk except the 

 tliui hvi)oderniic region and a stripe of pseudo-chorduid 

 tissue, still with its blood-vessel ventrally, the cuticle 

 enveloping all. The basement-tissue is slightly developed 

 in the ventral arch, but forms a well-marked layer dorsally, 

 fusing with the tough issue in the middle of the stalk, but 

 being better dittercntiated at the base of each external ridge, 

 a thin line of it running almost to the tip of tlie latter. 

 The section of a nerve (PI. VI. fig. 32, n.) occurs at each 

 outer angle and in the middle of the dorsal arch, the former 

 being outside the basement-tissue, the latter within it. In 

 the basal (proximal) or incipient condition of the stalk this 

 basement-tissue is less developed than distally, and the 

 relationships of the nerve therefore undergo changes. The 

 groove for the larger blood-vessel in some preparations 

 sinks more deeply into the ehordoid tissue. The pri)jocting 

 ends of the ovoid area of the opercular stalk assume' a 

 clavate outline and then disappear — that is to say, the ridge 

 on each side of the stalk ceases after the lateral filaments 

 of the stalk have separated. With the disappearance of the 

 lateral ridges the ehordoid tissue occupies in section the 

 entire area of the ovoid stalk, oidy a thin, barely visible, 

 belt of hypod. rm occurring under the cuticle. In some of 

 the sections the strands of the ehordoid tissue are arranged 

 in a somewhat radiate manner with the nuclei and cut ends 

 of fibres at the circumference, so that, when the hypoderra 

 and the cuticle are shed, such might be mistaken for 

 the modified hypoderm. Further, the blood-vessel is now 

 enveloped l)y the ehordoid tissue. Soon a differentiation in 

 tiie midst of this area appears as a smooth central region 

 from which lines radiate to the external margin. This 

 central region gradually increases distally, and the differ- 

 entiation of tiie radiating cells with the nuclei externally 

 gives it, in some preparations, the appearance of a hypo- 

 derm within a hypoderm as just mentioned ; and, moreover, 

 a ridge or pajjilla a])pears on one side of the actual cuticle 

 or hypoderm. The blood-channel is enclosed in the inner 

 area, and is large. The ventral hypoderm and cuticle 

 diminish and disappear, leaving what was the ehordoid area 

 and its central region, with the addition of a small patch, 

 isolated in cuticle, to repiesent the former envelope of the 

 stalk, and that soon vanishes.' Tims the enlarged opercular 

 stalk now consists of the tough cuticle, the modified coating 

 of the ehordoid area representing the hypoderm, with its 



