[203] 



Development of tbe Z^a^pole♦ 



By J. W. Gatehouse, F.LC. 



Part IV. 



Plates XIX., XX., XXI. 



TRx\CING back the development of the claspers described on 

 p. 151 (July), we find that, like the rest of the organs, they 

 too have undergone most decided alterations both in form and 

 structure, appearing first as a mere thickening of the epiblast, as 

 seen in Fig. i, Plate VII., and Figs, i and 2, Plate VIII. We 

 next find them showing an apparent invagination, or rather per- 

 haps a ring of dark, elongated, epiblastic cells, converging towards 

 a central opening, and resting upon what appears to be a 

 modification of cartilage. The whole is bounded posteriorly, 

 that is, on the inner side, with a single layer of cells continuous 

 with the mesoblastic layer, and indeed forming an integral part 

 of it (Fig. 5, Plate XXL). A section made two days later shows 

 the appearance given in Fig. 3, the two points shown in 

 Fig. 4 having advanced outwards and become double, whilst the 

 structure as a whole has also suffered a more complete longitu- 

 dinal division. The growth of this organ, as a whole, is 

 therefore seen to proceed much on the same lines as that which 

 occurs in simple cell division. These points ultimately coalesce, 

 the central division remaining, so that in their most perfect 

 development we arrive at the structure already figured in Plate 

 XIII., Fig. 7. 



In our last article some doubts were thrown out as to the true 

 function of this organ, and when we consider its epiblastic origin, 

 and also that muscles are produced from the mesoblast, we are 

 compelled to arrive at the conclusion that these so-called claspers 

 do not clasp, and also that they are not suctorial discs, for not 

 being muscular they cannot suck, and are therefore not homo- 

 logous with the suctorial mouth of the Cyclostomata, as seen in 

 the Lamprey. What, then, is this structure ? What are its 

 functions ? We have seen that at one period of its history it 

 certainly appears to partially support the animal in the water, but 



