502 PEOF. ALLMAN ON THE STEUCTUEB AND 



As yet no trace of muscular fibres can be detected in any part 

 of the tentacular sheath or of the alimentary canal. These are 

 formed at a later period ; but Mtsche has not been able to de- 

 termine from which of the two layers they are derived. The 

 fibres of the retractors and of the parieto-vaginal muscles arise 

 each out of a single cell of the endoderm — that is, of the outer 

 layer of the polypide-bud close to the spot where this is connected 

 with the wall of the cystid. 



Formation of the Statohlast. — Nitsche has further paid great 

 attention to the statoblasts of Alci/onella, and has given a very 

 complete account of their mode of formation, which he shows to 

 be a curious and complicated process*. 



These bodies, as is well known, consist of two parts, — a lenti- 

 cular disk enclosed in a chitinous envelope, and composed of 

 the material from which a young individual is to be developed ; 

 and a chitinous ring running round the edges of the disk, com- 

 posed of air-filled chambers and acting as a fioat. 



The statoblasts arise from a sausage-shaped body, which is 

 formed immediately below the outer epithelium of the funiculus, 

 and is composed of nucleolated nuclei with a small quantity of in- 

 tervening protoplasm. From this are constricted off one after 

 another small heaps of nuclei. These heaps continue to lie be- 

 tween the body of the funiculus and its epithelial layer. Each 

 of them represents a statoblast, and soon shows a division into 

 two halves by means of an equatorial furrow, so that it assumes 

 an appearance very like that of the vitellus of an ovum after its 

 first segmentation. In the next place that half which lies fur- 

 thest from the funiculus becomes excavated by a central cavity, 

 the nuclei which compose it arranging themselves in a single 

 layer on its periphery. This excavated half is destined for the 

 formation of the chitinous envelope with the float-ring, while the 

 other affords the material out of which the young animal is to be 

 developed. The former is termed by Nitsche the cystogenous 

 layer, the latter the formative mass. 



Protoplasm now collects round the nuclei, forming the walls of 

 the cystogenous layer, and converts them into true cells, which 

 become elongated prismatic, and assume the form of a cylinder 

 epithelium. An increase of protoplasm also occurs round the 

 nuclei of the formative mass. 



* Ai'cbiv fiir Anat. 1868. 



