Origin of the Perivisceral Coelom.

Soon after the embryo has assumed radial symmetry a change is observed in the texture of the mesenchyme, closely similar to that which has already been described for Kirk's ophiuroid (Fell, 1941). Between some of the cells of this tissue a process of splitting begins to take place, producing a number of intercellular cavities which are at first small. Serial sections reveal that there is another similarity to what occurs in Kirk's ophiuroid, in that this process of splitting begins in the outermost zone of mesenchyme, extending only subsequently to the more dorsal and central region. Thus, the mesenchyme immediately above the stomach is the last to be affected in this way. In Fig. 15 is illustrated a vertical section through the central region of an embryo in which the schizocoelous splitting is still at an early stage. There are a number of isolated cavities in the mesenchyme, and only in the outer and more lateral regions have they extended so as to link up with neighbouring clefts. Continuation of the process results in the fusion of all the spaces to produce the general body cavity, or perivisceral coelom. A vertical cross-section of one of the incipient arms of a young star, showing the earliest stage in the schizocoelous excavation of the perivisceral coelom, is seen in

Fig. 16.—Vertical section through the rudiment of the arm of a young star in which the coelom is appearing as splits between the cells of the mesenchyme.
Amb.C., ambulacral canal; Coel.Sp., schizocoelous intercellular splits forming the coelom; Ect., ectoderm; Mesench., mesenchyme; Pod., podium.

Fig. 16. Further extension of the splitting, with consequent linking up of the spaces produced, causes the coelom to take on what is essentially its adult relationship with the neighbouring organs. Its lining is at first irregular (Fig. 17), with cells and cytoplasmic bridges projecting into the lumen; but soon a coelomic epithelium is differentiated from the bounding mesenchyme cells, so that the perivisceral coelom takes on a more regular form (Fig. 18).

At this point it is proper to reiterate that one of the chief purposes of this study was to re-examine the statement made by Russo that the general coelom arises in Amphipholis squamata by splitting in mesenchyme (Russo, 1891). It can now be seen that (1) Russo's claim in this regard is completely confirmed, and that he thus is the original discoverer of this mode of origin of the perivisceral coelom in an echinoderm; and (2) the mode of origin of the perivisceral coelom in A. squamata differs but little from what has