COIMPARATIVE ANATOMY. 29 



sheath is probably formed by seven or eight cells, slender in form, indistinct 

 in outline, with dense nuclei and attenuated bases which are imbedded in the 

 hypodermis proper. The.y entirely surround the basal cell, and distally their 

 membranes become continuous with the spicule sheath, which as Plate 36, fig. 5, 

 shows, is thus interrupted a short distance above the base of the spine. 



At a relatively early stage in the development of the spine a minute cell 

 (Plate 36, fig. 11) may be detected between the basal cell proper and the spicule 

 sheath. At first it appears to be connected with the deeper portions of the 

 hypodermis by a single stalk that passes to one side of the basal cell; but in 

 later stages such a connection disappears and the cell in question becomes closely 

 applied to the base of the spine. It appears to be responsible for the formation 

 of the spicule sheath immediatelj' above it and in the following way for the 

 cavity of the spicule itself. At first the cuticle above this small basal sheath 

 cell is of uniform thickness and the lime salts, deposited presumably by the basal 

 cell proper completely fill the spicule sheath, but very soon a minute knob-like 

 elevation appears on the basal part of the sheath, and, perhaps owing to this 

 increased thickness of the cuticle, it interferes with the deposition of calcareous 

 material, for from this time on a cavity develops in the spine that in size and 

 position corresponds to the cuticular knob. As the latter increases in size the 

 spicule cavity enlarges, and when in later stages there is a decrease the cavity 

 becomes proportionately narrowed until both finally disappear together. 



In the earliest stages of its existence the long axis of the spine is at right 

 angles to the hypodermis, but as development progresses it becomes more and 

 more inclined until it reaches the final horizontal position. This rotation is 

 probably due in large measure to the unequal elongation of the attached cells, 

 while the continual advance of the free tip of the spine through the surrounding 

 cuticle is due to the addition of new material basally. During this whole forma- 

 tive period and after its completion the entire spicule is migrating also toward 

 the free surface of the body. Hubrecht and several other subseciuent observers 

 consider that this movement is caused by the continual cuticular current, so 

 to speak, brought about by the perpetual addition of new material in contact 

 with the hypodermis, a belief with which I concur for on any other supposition 

 it would be difficult to account for the perfectly vertical, unbent position of the 

 slender remnant of the matrix cells in late stages. 



Until the spine has been carried outward for a distance equal to one fourth 

 or one third the thickness of the cuticle the matrix cells retain their usual con- 

 nections and generally are fairly distinct though showing more or less shrinkage 



