Figs. 79 and 80. Transverse section of amphioxus-larvae. Figs. 79 and 80.—Transverse section of amphioxus-larvæ. (From _Hatschek._) Fig. 79 at the commencement of cœlom formation (still without segments), Fig. 80 at the stage with four primitive segments. _ak, ik, mk_ outer, inner, and middle germinal layer, _hp_ horn plate, _mp_ medullary plate, _ch_ chorda, * and * disposition of the cœlom-pouches, _lh_ body-cavity.)
The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the _Monoxenia_ which I described (Chapter VIII, Fig. 29). This oval, uni-axial cup-larva (circular in section) becomes bilateral (or tri-axial) by the growth of a couple of cœlom-pouches from the primitive gut (Figs. 76, 77). To the right and left a sac-shaped fold appears towards the top pole (where the permanent mouth, _m,_ afterwards arises). The two sacs are at first separated by a couple of folds of the entoderm (Fig. 76 _ pv_), and are still connected with the primitive gut by wide apertures; they also communicate for a short time with the dorsal side (Fig. 77 _d_). Soon, however, the cœlom-pouches completely separate from each other and from the primitive gut; at the same time they enlarge so much that they close round the primitive gut (Fig. 78). But in the middle line of the dorsal and ventral sides the pouches remain separated, their approaching walls joining here to form a thin vertical partition, the mesentery (_dm_ and _vm_). Thus _ Sagitta_ has throughout life a double body-cavity (Fig. 78 _ lk_), and the gut is fastened to the body-wall both above and below by a mesentery—below by the ventral mesentery (_vm_), and above by the dorsal mesentery (_dm_). The inner layer of the two cœlom-pouches (_mv_) attaches itself to the entoderm (_ik_), and forms with it the visceral wall. The outer layer (_mp_) attaches itself to the ectoderm (_ak_), and forms with it the outer body-wall. Thus we have in _Sagitta_ a perfectly clear and simple illustration of the original cœlomation of the enterocœla. This palingenetic fact is the more important, as the greater part of the two body-cavities in _Sagitta_ changes afterwards into sexual glands—the fore or female part into a pair of ovaries, and the hind or male part into a pair of testicles.